Log Message:
graphics/gdk-pixbuf2: Make this cross-compile.

1. Break introspection out into an option, default off for cross
   builds, default on for native builds (so no change to native
   builds).

2. Don't play LD_LIBRARY_PATH games at build time in cross builds; it
   won't do anything useful.

3. For some reason the thumbnailer is disabled upstream in cross
   builds, which seems sketchy, but maybe it's not that important.