Check for __readfsdword at configure-time

Recent versions of mingw-w64 have moved this definition to a header
included by this file, so checking simply for GCC would cause a multiple
definition error. We should instead check for this function's
availability at configure-time.

These changes are released under MIT/X11 license.
Merge pull request #813 from ColdPie1/mingw64fix

Check for __readfsdword at configure-time