Rework handling of REBUILD_PNGS, so that we also don't try to REBUILD_PNGS

Rework handling of REBUILD_PNGS, so that we also don't try to REBUILD_PNGS

Mon Aug 18 10:51:57 2003  Owen Taylor  <otaylor@redhat.com>

        * configure.in: Rework handling of REBUILD_PNGS,
        so that we also don't try to REBUILD_PNGS when
        cross-compiling and there is no gdk-pixbuf-csource.
        But give an error if gtk/stock-icons/gtkstockpixbufs.h
        is not in the source tree. (Note that REBUILDS_PNG
        was set backwards before, which is why you always
        had to manually recreate gtkstockpixbufs.h!)