don't check for libjpeg,libtiff and libpng when building wxMGL,

don't check for libjpeg,libtiff and libpng when building wxMGL,
they are part of libmgl and we want to reuse them (for two reasons: one, they are
dynamically loaded at runtime; and two, we get symbols conflicts otherwise

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@13816 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775