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
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