Build libgltf (and freetype) for iOS, too

They won't be used for now, but just to enforce portability of the
libgltf code a bit.

Change-Id: Ie5d2b9125eb0f4a2e432dcb8b55d5f494a5ed34d
No fpicker needed for non-DESKTOP surely?

Sure, not a lot of code, but best to not even compile it, to catch
eventual usage attempts.

Change-Id: I7462ea9a5fbe6080fcec2f424d1a1b80ad0afe21