Restore embedding manifest when using MinGW.

Restore embedding manifest when using MinGW.

The changes of r73483 broke inclusion of the manifest in the programs built
using MinGW because wxUSE_RC_MANIFEST was never defined. Somehow nobody
complained about it but this resulted in using comctl32.dll 5.80 instead of
6.0 and e.g. problems with toolbar appearance (see #13512).

Do use the manifest by default with MinGW and, in fact, all the other
compilers if any other ones still work, as only MSVC is known to embed the
manifest automatically and we take care of it separately.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74509 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775