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/libmariadb/CMakeLists.txt
+++ b/libmariadb/CMakeLists.txt
@@ -417,7 +417,7 @@ ELSE()
   SET_TARGET_PROPERTIES(libmariadb PROPERTIES LINKER_LANGUAGE C)
 ENDIF()
 
-TARGET_LINK_LIBRARIES(libmariadb LINK_PRIVATE ${SYSTEM_LIBS})
+TARGET_LINK_LIBRARIES(libmariadb LINK_PRIVATE ${SYSTEM_LIBS} ${LIBUCONTEXT_POSIX} ${LIBUCONTEXT})
 
 SIGN_TARGET(libmariadb)
 
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -230,6 +230,14 @@ 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} ${LIBZ}
     ${LIBSOCKET} ${CMAKE_DL_LIBS} ${LIBM} ${LIBPTHREAD})
--- a/include/ma_config.h.in
+++ b/include/ma_config.h.in
@@ -28,6 +28,7 @@
 #cmakedefine HAVE_SYS_UN_H 1
 #cmakedefine HAVE_UNISTD_H 1
 #cmakedefine HAVE_UCONTEXT_H 1
+#cmakedefine HAVE_LIBUCONTEXT_POSIX 1
 
 /*
  * function definitions - processed in LibmysqlFunctions.txt 
--- a/include/ma_context.h
+++ b/include/ma_context.h
@@ -31,7 +31,7 @@
 #define MY_CONTEXT_USE_X86_64_GCC_ASM
 #elif defined(__GNUC__) && __GNUC__ >= 3 && defined(__i386__)
 #define MY_CONTEXT_USE_I386_GCC_ASM
-#elif defined(HAVE_UCONTEXT_H)
+#elif defined(HAVE_UCONTEXT_H) || defined(HAVE_LIBUCONTEXT_POSIX)
 #define MY_CONTEXT_USE_UCONTEXT
 #else
 #define MY_CONTEXT_DISABLE