diff --git a/include/site/cache b/include/site/cache
index 7a19181c6fe..3b871602129 100644
--- a/include/site/cache
+++ b/include/site/cache
@@ -21,17 +21,47 @@ if [ -n "$CONFIGURE_CACHE_FILE" ]; then
 	# A change of the configure arguments gives a new file name. Drop the
 	# file of the previous arguments so that the directory holds one cache
 	# per package.
-	for configure_cache_stale in "$configure_cache_dir"/*.cache; do
+	for configure_cache_stale in "$configure_cache_dir"/*.cache "$configure_cache_dir"/*.cache.sum; do
 		[ -f "$configure_cache_stale" ] || continue
-		[ "$configure_cache_stale" = "$CONFIGURE_CACHE_FILE" ] || rm -f "$configure_cache_stale"
+		[ "${configure_cache_stale%.sum}" = "$CONFIGURE_CACHE_FILE" ] || rm -f "$configure_cache_stale"
 	done
 
+	# An autoreconf run with a new autoconf, automake or libtool, or a new
+	# patch, gives a new configure script whose checks can mean something
+	# else under the same cache variable name.
+	configure_cache_sum=$(cksum < "${as_myself:-$0}")
+	if [ "$configure_cache_sum" != "$(cat "$CONFIGURE_CACHE_FILE.sum" 2>/dev/null)" ]; then
+		rm -f "$CONFIGURE_CACHE_FILE"
+		printf '%s\n' "$configure_cache_sum" > "$CONFIGURE_CACHE_FILE.sum"
+	fi
+
+	# configure aborts when a precious variable such as LIBS or CPP differs
+	# from the value in the cache, and it aborts again on every later run.
+	# Make the same comparison here and start from a cold cache instead.
+	if [ -f "$CONFIGURE_CACHE_FILE" ] && ! (
+		set -f
+		. "$CONFIGURE_CACHE_FILE"
+		for ac_var in $ac_precious_vars; do
+			eval "old_set=\$ac_cv_env_${ac_var}_set new_set=\$ac_env_${ac_var}_set"
+			[ "$old_set" = "$new_set" ] || exit 1
+			eval "set -- \$ac_cv_env_${ac_var}_value"
+			old_val=$*
+			eval "set -- \$ac_env_${ac_var}_value"
+			[ "$old_val" = "$*" ] || exit 1
+		done
+	); then
+		rm -f "$CONFIGURE_CACHE_FILE"
+	fi
+
 	# A cached program path outlives a wipe of staging_dir, so it can name a
 	# tool that is gone, or miss one that the build installed since. Drop
 	# those answers and let configure search PATH again. A search runs no
-	# compiler, and the compiler is where the time goes.
+	# compiler, and the compiler is where the time goes. The libtool and gmp
+	# answers hold the path of the staged sed and m4.
 	if [ -f "$CONFIGURE_CACHE_FILE" ]; then
-		grep -v -e '^ac_cv_path_' -e '^ac_cv_prog_' \
+		grep -v -e '^[a-z]*_cv_path_' -e '^ac_cv_prog_' -e '^gmp_cv_prog_' \
+			-e '^lt_cv_sys_global_symbol_' -e '^lt_cv_truncate_bin=' \
+			-e '^ac_cv_AM_TEST_RUNNER_SHELL=' \
 			"$CONFIGURE_CACHE_FILE" > "$CONFIGURE_CACHE_FILE.tmp"
 		mv -f "$CONFIGURE_CACHE_FILE.tmp" "$CONFIGURE_CACHE_FILE"
 	fi
@@ -40,5 +70,5 @@ if [ -n "$CONFIGURE_CACHE_FILE" ]; then
 
 	# 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
+	unset CONFIGURE_CACHE_FILE configure_cache_dir configure_cache_stale configure_cache_sum
 fi
diff --git a/rules.mk b/rules.mk
index 2eba07279e3..3adf4ec2fc0 100644
--- a/rules.mk
+++ b/rules.mk
@@ -220,8 +220,8 @@ endif
 #
 # 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.
+# arguments and build flags for both. include/site/cache also starts from a
+# cold cache when the configure script or a precious variable changed.
 #
 # 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