Enable building libgdiplus with freetype 2.5.1 and later

Patch authored by Ryan Schmidt <xamarin@ryandesign.com>

Xamarin bug 16521 (http://bugzilla.xamarin.com/show_bug.cgi?id=16521)

Currently a problem on trusty and jessie

Change-Id: I39f140801213dd256ab17b15b7314c8bd50bf480