Bug #33791736 The POST_BUILD TARGET libmysql_api_test may fail when bulding with Xcode

Post-push fix: use MY_ADD_CUSTOM_TARGET to silence msbuild:
    Warning MSB8065: File not created
    Custom build for ... succeeded, but specified output ... has not been created.

Change-Id: Id9bbdc6263694b0a50f8bc460e796dd396ff0309

Author: Tor Didriksen

libmysql/CMakeLists.txt

@@ -427,7 +427,7 @@ IF(HAS_WARN_FLAG)
 ENDIF()
 
 # Verify that libmysql_api_test runs OK
-ADD_CUSTOM_TARGET(run_libmysql_api_test ALL
+MY_ADD_CUSTOM_TARGET(run_libmysql_api_test ALL
   DEPENDS libmysql_api_test
   COMMAND libmysql_api_test
   > ${CMAKE_CURRENT_BINARY_DIR}/libmysql_api_test.out