Don't use ancient -no-cpp-precomp flag on OS X.

Don't use ancient -no-cpp-precomp flag on OS X.

See http://trac.wxwidgets.org/ticket/14029 for a discussion of the
problems it causes when using non-Apple compiler. Even for Apple's GCC,
it does nothing these days and we no longer support OS X 10.3, so
there's no point in continuing to use it.

Fixes #14029.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71314 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775