diff --git a/include/host-build.mk b/include/host-build.mk
index b459c0ab18c..a0419d8e37a 100644
--- a/include/host-build.mk
+++ b/include/host-build.mk
@@ -109,9 +109,12 @@ HOST_MAKE_FLAGS =
 
 HOST_CONFIGURE_CMD = $(BASH) ./configure
 
-ifeq ($(HOST_OS),Darwin)
-  HOST_CONFIG_SITE:=$(INCLUDE_DIR)/site/darwin
-endif
+HOST_CONFIG_SITE_BASE:=$(if $(filter Darwin,$(HOST_OS)),$(INCLUDE_DIR)/site/darwin)
+HOST_CONFIG_SITE:=$(INCLUDE_DIR)/site/cache
+# Set HOST_CONFIGURE_CACHE:=1 to keep the autoconf result cache of this host
+# build between builds. Read the note above PKG_CONFIGURE_CACHE in package.mk
+# before you opt a package in.
+HOST_CONFIGURE_CACHE_FILE = $(CONFIGURE_CACHE_BASE)/$(notdir $(BUILD_DIR_HOST))-$(host_cc_id)/$(notdir $(HOST_BUILD_DIR))/$(call strhash,$(HOST_CONFIGURE_ARGS) $(HOST_CONFIGURE_VARS)).cache
 
 define Host/Configure/Default
 	$(if $(HOST_CONFIGURE_PARALLEL),+)(cd $(HOST_BUILD_DIR)/$(3); \
@@ -170,6 +173,7 @@ define Host/Exports/Default
   $(1) : export PKG_CONFIG_LIBDIR=$$(HOST_BUILD_PREFIX)/lib/pkgconfig
   $(1) : export GIT_CEILING_DIRECTORIES=$$(BUILD_DIR_HOST)
   $(if $(HOST_CONFIG_SITE),$(1) : export CONFIG_SITE:=$(HOST_CONFIG_SITE))
+  $(if $(HOST_CONFIG_SITE_BASE),$(1) : export CONFIG_SITE_BASE:=$(HOST_CONFIG_SITE_BASE))
   $(if $(IS_PACKAGE_BUILD),$(1) : export PATH=$$(TARGET_PATH_PKG))
 endef
 Host/Exports=$(Host/Exports/Default)
@@ -192,6 +196,7 @@ ifndef DUMP
 	$(call BuildTimeLog,end,prepare)
 
   $(call Host/Exports,$(HOST_STAMP_CONFIGURED))
+  $(if $(and $(CONFIGURE_CACHE_BASE),$(HOST_CONFIGURE_CACHE)),$(HOST_STAMP_CONFIGURED) : export CONFIGURE_CACHE_FILE:=$(HOST_CONFIGURE_CACHE_FILE))
   $(HOST_STAMP_CONFIGURED): $(HOST_STAMP_PREPARED)
 	$(call BuildTimeLog,begin,configure)
 	$(foreach hook,$(Hooks/HostConfigure/Pre),$(call $(hook))$(sep))
diff --git a/include/package.mk b/include/package.mk
index fe213f80411..4ff79658f66 100644
--- a/include/package.mk
+++ b/include/package.mk
@@ -188,11 +188,19 @@ include $(INCLUDE_DIR)/autotools.mk
 _pkg_target:=$(if $(QUILT),,.)
 
 override MAKEFLAGS=
-CONFIG_SITE:=$(INCLUDE_DIR)/site/$(ARCH)
+CONFIG_SITE_BASE:=$(INCLUDE_DIR)/site/$(ARCH)
+CONFIG_SITE:=$(INCLUDE_DIR)/site/cache
+# Set PKG_CONFIGURE_CACHE:=1 to keep the autoconf result cache of this package
+# between builds. Opt in only after you check that the package configures the
+# same way with a warm cache as it does without one. A configure script that
+# changes CPPFLAGS or CFLAGS inside an AC_CACHE_VAL body is not safe: a warm
+# cache restores the value and skips the change. Compare the generated
+# Makefile, not only config.h, because that is where the difference shows.
+PKG_CONFIGURE_CACHE_FILE = $(CONFIGURE_CACHE_BASE)/$(TOOLCHAIN_DIR_NAME)/$(notdir $(PKG_BUILD_DIR))/$(call strhash,$(CONFIGURE_ARGS) $(CONFIGURE_VARS)).cache
 CUR_MAKEFILE:=$(filter-out Makefile,$(firstword $(MAKEFILE_LIST)))
 SUBMAKE:=$(NO_TRACE_MAKE) $(if $(CUR_MAKEFILE),-f $(CUR_MAKEFILE))
 PKG_CONFIG_PATH=$(STAGING_DIR)/usr/lib/pkgconfig:$(STAGING_DIR)/usr/share/pkgconfig
-unexport QUIET CONFIG_SITE
+unexport QUIET CONFIG_SITE CONFIG_SITE_BASE
 
 ifeq ($(DUMP)$(filter prereq clean refresh update,$(MAKECMDGOALS)),)
   ifneq ($(if $(QUILT),,$(CONFIG_AUTOREBUILD)),)
@@ -224,6 +232,7 @@ define Build/Exports/Default
   $(1) : export STAGING_PREFIX=$$(STAGING_DIR)/usr
   $(1) : export PATH=$$(TARGET_PATH_PKG)
   $(1) : export CONFIG_SITE:=$$(CONFIG_SITE)
+  $(1) : export CONFIG_SITE_BASE:=$$(CONFIG_SITE_BASE)
   $(1) : export PKG_CONFIG_PATH:=$$(PKG_CONFIG_PATH)
   $(1) : export PKG_CONFIG_LIBDIR:=$$(PKG_CONFIG_PATH)
   $(1) : export GIT_CEILING_DIRECTORIES:=$$(BUILD_DIR)
@@ -258,6 +267,7 @@ define Build/CoreTargets
 	$(call BuildTimeLog,end,prepare)
 
   $(call Build/Exports,$(STAMP_CONFIGURED))
+  $(if $(and $(CONFIGURE_CACHE_BASE),$(PKG_CONFIGURE_CACHE)),$(STAMP_CONFIGURED) : export CONFIGURE_CACHE_FILE:=$(PKG_CONFIGURE_CACHE_FILE))
   $(STAMP_CONFIGURED): $(STAMP_PREPARED) $(STAMP_CONFIGURED_DEPENDS)
 	$(call BuildTimeLog,begin,configure)
 	rm -f $(STAMP_CONFIGURED_WILDCARD)
diff --git a/include/site/cache b/include/site/cache
new file mode 100644
index 00000000000..7a19181c6fe
--- /dev/null
+++ b/include/site/cache
@@ -0,0 +1,44 @@
+#!/bin/sh
+#
+# Points the autoconf result cache at the file that the build system keeps for
+# this package. Set CONFIGURE_CACHE_DIR to switch this on; see rules.mk.
+#
+# configure sources the site scripts before it loads the cache, so setting
+# cache_file here has the same effect as --cache-file, without a configure
+# argument that a hand written configure script would choke on.
+
+# Autoconf before 2.70 reads CONFIG_SITE as a single path name, so the site
+# script of the architecture cannot be a second word in it. Load it from here.
+if [ -n "$CONFIG_SITE_BASE" ] && [ -r "$CONFIG_SITE_BASE" ]; then
+	. "$CONFIG_SITE_BASE"
+fi
+
+if [ -n "$CONFIGURE_CACHE_FILE" ]; then
+	configure_cache_dir=${CONFIGURE_CACHE_FILE%/*}
+
+	mkdir -p "$configure_cache_dir" 2>/dev/null
+
+	# A change of the configure arguments gives a new file name. Drop the
+	# file of the previous arguments so that the directory holds one cache
+	# per package.
+	for configure_cache_stale in "$configure_cache_dir"/*.cache; do
+		[ -f "$configure_cache_stale" ] || continue
+		[ "$configure_cache_stale" = "$CONFIGURE_CACHE_FILE" ] || rm -f "$configure_cache_stale"
+	done
+
+	# A cached program path outlives a wipe of staging_dir, so it can name a
+	# tool that is gone, or miss one that the build installed since. Drop
+	# those answers and let configure search PATH again. A search runs no
+	# compiler, and the compiler is where the time goes.
+	if [ -f "$CONFIGURE_CACHE_FILE" ]; then
+		grep -v -e '^ac_cv_path_' -e '^ac_cv_prog_' \
+			"$CONFIGURE_CACHE_FILE" > "$CONFIGURE_CACHE_FILE.tmp"
+		mv -f "$CONFIGURE_CACHE_FILE.tmp" "$CONFIGURE_CACHE_FILE"
+	fi
+
+	cache_file=$CONFIGURE_CACHE_FILE
+
+	# A configure script that runs further configure scripts must not hand
+	# this cache to them. Their checks run with different flags.
+	unset CONFIGURE_CACHE_FILE configure_cache_dir configure_cache_stale
+fi
diff --git a/rules.mk b/rules.mk
index adae18b97fc..db5b2a91c63 100644
--- a/rules.mk
+++ b/rules.mk
@@ -206,6 +206,33 @@ else
   BuildTimeLog =
 endif
 
+# A package that sets PKG_CONFIGURE_CACHE or HOST_CONFIGURE_CACHE keeps the
+# autoconf result cache of its configure checks, so a package that is cleaned
+# and built again reads the answers instead of running the checks. The cache
+# lives in tmp/, which dirclean removes, so a full reset of the tree starts
+# from cold answers.
+#
+# The path covers everything that changes an answer: the toolchain for a target
+# package, the host compiler for a host package, and a hash of the configure
+# arguments and build flags for both. autoconf refuses a cache that it wrote
+# with different flags on top of that.
+#
+# A package must opt in, because a cache is only correct for a configure script
+# that keeps its side effects outside the AC_CACHE_VAL body. ncurses appends
+# -D_XOPEN_SOURCE to CPPFLAGS inside that body, so a cached run restores the
+# value, skips the append and then fails to compile. Read the note above
+# PKG_CONFIGURE_CACHE in package.mk before you opt a package in.
+#
+# Set CONFIGURE_CACHE_DIR to a path of your own to keep the cache elsewhere, or
+# to the empty string to run every check again.
+ifeq ($(origin CONFIGURE_CACHE_DIR),undefined)
+  CONFIGURE_CACHE_DIR:=$(TMP_DIR)/configure-cache
+endif
+ifneq ($(CONFIGURE_CACHE_DIR),)
+  CONFIGURE_CACHE_BASE:=$(if $(filter /%,$(CONFIGURE_CACHE_DIR)),$(CONFIGURE_CACHE_DIR),$(TOPDIR)/$(CONFIGURE_CACHE_DIR))
+  strhash=$(shell printf '%s' '$(subst ','\'',$(1))' | $(MKHASH) md5)
+endif
+
 BUILD_DIR_HOST:=$(if $(IS_PACKAGE_BUILD),$(BUILD_DIR_BASE)/hostpkg,$(BUILD_DIR_BASE)/host)
 STAGING_DIR_HOST:=$(abspath $(STAGING_DIR)/../host)
 STAGING_DIR_HOSTPKG:=$(abspath $(STAGING_DIR)/../hostpkg)
@@ -359,6 +386,14 @@ export TARGET_CXX_NOCACHE
 export HOSTCC_NOCACHE
 export HOSTCXX_NOCACHE
 
+# A new host compiler behind an unchanged name changes what a configure check
+# answers, and autoconf does not detect that. gcc 14 turned an implicit function
+# declaration into an error, which changed many of those answers. The configure
+# cache of a host package is therefore keyed on the compiler as well. prereq
+# builds mkhash and links the compiler into staging_dir, so both exist by the
+# time a configure recipe asks for this.
+host_cc_id = $(if $(filter undefined,$(origin __host_cc_id)),$(eval __host_cc_id:=$(shell $(HOSTCC_NOCACHE) --version 2>/dev/null | head -1 | $(MKHASH) md5)))$(__host_cc_id)
+
 ifneq ($(CONFIG_CCACHE),)
   TARGET_CC:= ccache $(TARGET_CC)
   TARGET_CXX:= ccache $(TARGET_CXX)