Don't warn about wxMetaFile in configure by default.

Don't warn about wxMetaFile in configure by default.

Only warn if an explicit --enable-metafile was given, otherwise just turn it
on only for the platforms that do support it, similarly to what was done for
--enable-hotkey in the previous commit.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68969 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775