autoconf: Reject --enable-xml --disable-png
The xml surface depends on png. Before this commit, configuring with autogen.sh --disable-png --disable-svg --enable-xml resulted in a failing build:
src/cairo-xml-surface.c: In function ‘_cairo_xml_emit_image’: src/cairo-xml-surface.c:673:14: error: implicit declaration of function ‘cairo_surface_write_to_png_stream’
After this commit, configure instead rejects this combination with:
checking whether cairo's xml surface backend feature could be enabled... no (requires --enable-png) configure: error: xml surface backend feature could not be enabled
Noticed-at: !67 (comment 644347) Signed-off-by: Uli Schlachter psychon@znc.in
CC @xclaesse because of !67 (comment 644235) (but feel free to just ignore this CC)