CMake: fix CHECK_OBJC_SOURCE_COMPILES to work with GCC (thanks, Ivan!).

Apparently -ObjC works with Clang, but -x objective-c works with both.

Fixes Bugzilla #2392.

cmake/macros.cmake

@@ -66,7 +66,7 @@ endmacro()
 
 macro(CHECK_OBJC_SOURCE_COMPILES SOURCE VAR)
   set(PREV_REQUIRED_DEFS "${CMAKE_REQUIRED_DEFINITIONS}")
-  set(CMAKE_REQUIRED_DEFINITIONS "-ObjC ${PREV_REQUIRED_DEFS}")
+  set(CMAKE_REQUIRED_DEFINITIONS "-x objective-c ${PREV_REQUIRED_DEFS}")
   CHECK_C_SOURCE_COMPILES(${SOURCE} ${VAR})
   set(CMAKE_REQUIRED_DEFINITIONS "${PREV_REQUIRED_DEFS}")
 endmacro()