fix ucontext configure check

musl ships the header for other purposes, but makecontext is not
implemented. fix the check to detect if makecontext is implemented
before enabling code using it.

configure.cmake

@@ -1111,6 +1111,9 @@ CHECK_INCLUDE_FILE(ucontext.h HAVE_UCONTEXT_H)
 IF(NOT HAVE_UCONTEXT_H)
   CHECK_INCLUDE_FILE(sys/ucontext.h HAVE_UCONTEXT_H)
 ENDIF()
+IF(HAVE_UCONTEXT_H)
+  CHECK_FUNCTION_EXISTS(makecontext HAVE_UCONTEXT_H)
+ENDIF()
 
 CHECK_STRUCT_HAS_MEMBER("struct timespec" tv_sec "time.h" STRUCT_TIMESPEC_HAS_TV_SEC)
 CHECK_STRUCT_HAS_MEMBER("struct timespec" tv_nsec "time.h" STRUCT_TIMESPEC_HAS_TV_NSEC)