libmariadb: Fix async api by linking to libucontext
The asynchronous API of libmariadb uses cooperative multi threading
by using the system calls
  * makecontext
  * swapcontext
  * getcontext
  * setcontext
of the ucontext.h C-API.

Thus additionally link libmariadb to libucontext which is a library
providing these system calls on platforms not supporting them out of
the box - like musl based platforms.
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -263,11 +263,19 @@ IF(UNIX)
   SEARCH_LIBRARY(LIBPTHREAD pthread_getspecific "pthread;pthreads")
   SEARCH_LIBRARY(LIBNSL gethostbyname_r "nsl_r;nsl")
   SEARCH_LIBRARY(LIBSOCKET setsockopt socket)
+  SEARCH_LIBRARY(LIBUCONTEXT libucontext_swapcontext libucontext.a)
+  SEARCH_LIBRARY(LIBUCONTEXT_POSIX swapcontext libucontext_posix.a)
+  IF (NOT HAVE_LIBUCONTEXT_POSIX OR NOT HAVE_LIBUCONTEXT)
+    UNSET(HAVE_LIBUCONTEXT)
+    UNSET(LIBUCONTEXT)
+    UNSET(HAVE_LIBUCONTEXT_POSIX)
+    UNSET(LIBUCONTEXT_POSIX)
+  ENDIF()
   FIND_PACKAGE(Threads)
   SET(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} ${LIBNSL} ${LIBBIND} ${LIBICONV} ${ZLIB_LIBRARY} 
-      ${LIBSOCKET} ${CMAKE_DL_LIBS} ${LIBM} ${LIBPTHREAD})
+      ${LIBSOCKET} ${LIBUCONTEXT} ${LIBUCONTEXT_POSIX} ${CMAKE_DL_LIBS} ${LIBM} ${LIBPTHREAD})
   SET(SYSTEM_LIBS ${SYSTEM_LIBS} ${LIBNSL} ${LIBBIND} ${LIBICONV} 
-    ${LIBSOCKET} ${CMAKE_DL_LIBS} ${LIBM} ${LIBPTHREAD})
+    ${LIBSOCKET} ${LIBUCONTEXT} ${LIBUCONTEXT_POSIX} ${CMAKE_DL_LIBS} ${LIBM} ${LIBPTHREAD})
   #remove possible dups from required libraries
   LIST(LENGTH CMAKE_REQUIRED_LIBRARIES rllength)
   IF(${rllength} GREATER 0)
--- a/include/ma_config.h.in
+++ b/include/ma_config.h.in
@@ -29,6 +29,7 @@
 #cmakedefine HAVE_SYS_UN_H 1
 #cmakedefine HAVE_UNISTD_H 1
 #cmakedefine HAVE_UCONTEXT_H 1
+#cmakedefine HAVE_LIBUCONTEXT_POSIX 1
 #cmakedefine HAVE_BOOST_CONTEXT_H 1
 
 /*
--- a/include/ma_context.h
+++ b/include/ma_context.h
@@ -61,7 +61,7 @@
 #define MY_CONTEXT_USE_AARCH64_GCC_ASM
 #elif defined(HAVE_BOOST_CONTEXT_H)
 #define MY_CONTEXT_USE_BOOST_CONTEXT
-#elif defined(HAVE_UCONTEXT_H)
+#elif defined(HAVE_UCONTEXT_H) || defined(HAVE_LIBUCONTEXT_POSIX)
 #define MY_CONTEXT_USE_UCONTEXT
 #else
 #define MY_CONTEXT_DISABLE