Work around wxFinite() definition conflict with <cmath>.

Using the GNU libstdc++ or Clang's libc++ implementations of the
standard library, <cmath> inclusion undefines the isfinite macro,
resulting in compilation errors when wxFinite() is used.  <cmath> can be
included unintentionally, e.g.  wx/hashmap.h may result in its inclusion
when using STD containers.

Work around this mess by checking if one of these <cmath>
implementations were already included and using std::isfinite() in that
case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74456 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775