Use std::isfinite() for wxFinite() for C++11 compilers.

This should fix compilation with MinGW 4.8.1 cross-compiler in C++11 mode as
it doesn't seem to provide finite() any more then.

Also, only defined wxFinite() and wxIsNaN() for C++, not C. This makes the
checks for __cplusplus >= 201103 simpler and is consistent with how
wxIsSameDouble() and wxRound() were already only defined for C++ (this is also
the source of most of the changes in this diff, viewing it ignoring whitespace
will show the only real changes).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74837 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775