"#error <my_config.h>" and third-party libraries

The safety check in my_config.h ensures that this file is always included before system headers.

But when my_config.h is installed, this breaks compilation of various third-party projects. In particular, mysql-python doesn't compile anymore.

While technically forcing my_config.h first is correct and mysql-python doesn't break by a sheer luck and, we still cannot force all third-party projects to include files correctly.

This check has to be relaxed.