Fix Lua for CMake with clang

When using CMake with clang, the wesnoth_lua_config.h was omitted causing problems with exception handling (among other unreported issues).

This closes Issue #1669

src/CMakeLists.txt

@@ -283,10 +283,10 @@ if(UNIX AND NOT CMAKE_COMPILER_IS_GNUCXX)
 	# version of cmake will be able to this for us.
 	#
 	# Also silence some Clang specific warnings due to extra parenthesis in if statements when comparing instead
-	set_source_files_properties(
-		${libwesnoth-lua_STAT_SRC}
-		PROPERTIES COMPILE_FLAGS
-			"-x c++ -Wno-parentheses-equality"
+	set_property(SOURCE
+		SOURCE ${libwesnoth-lua_STAT_SRC}
+		APPEND_STRING PROPERTY COMPILE_FLAGS
+			" -x c++ -Wno-parentheses-equality"
 	)
 endif(UNIX AND NOT CMAKE_COMPILER_IS_GNUCXX)