wxWidgets 2.8 defines file dialog enums as wxFD_... while 2.6 used wx_...
This patch detects whether or not the wxFD_... enums are available and
uses them; otherwise the old enums are used.
On Linux wxWidgets 2.8.0 and newer uses the GTK combo boxes which suck. A lot.
Therefore mmg uses wxBitmapComboBoxes for wxWidgets >= 2.8.0 on Linux and normal
wxComboBoxes in all other cases. wxBitmapComboBoxes are still drawn by wxWidgets
itself (just like wxComboBoxes before 2.8.0) and offer much better functionality.
Fix for bug 339.
windres is not needed for non-Windows builds and should
therefore only be checked if we are cross-compiling.
Otherwise warnings or errors could confuse the user.
The windres tool used for cross-compiled mingw builds
should be detected by configure. Until now it was assumed
that it was simply called "windres" and not prefixed
like the other compiler components (e.g. i386-mingw32msvc-gcc).
