CompileCheck: Fix check_flag for MSVC

cmake/CompileCheck.cmake

@@ -117,7 +117,11 @@ endfunction(check_compile)
 
 function(check_flag RESULT FLAG TYPE)
 	set(compile_test_file "${CMAKE_CURRENT_BINARY_DIR}/compile_flag_test.cpp")
-	file(WRITE ${compile_test_file} "__attribute__((const)) int main(){ return 0; }\n")
+	if(MSVC)
+		file(WRITE ${compile_test_file} "int main(){ return 0; }\n")
+	else()
+		file(WRITE ${compile_test_file} "__attribute__((const)) int main(){ return 0; }\n")
+	endif()
 	check_compile(result "${compile_test_file}" "${FLAG}" "${TYPE} flag")
 	set(${RESULT} "${result}" PARENT_SCOPE)
 endfunction(check_flag)