gdk-pixbuf: small cleanups

gdk-pixbuf: small cleanups

Update homepage, as the wiki is mostly dead links.

Optionally build the manpages.

Remove the gdk-pixbuf-xlib package, as this was removed in 2.42[1]

Enable/disable building the tests, not just installing the tests, to
save build time if they're not needed.

Fix some indentation which was using the wrong spacing.

[1] gdk-pixbuf 3362e94c2 ("Remove obsolete Xlib helper library")

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>