Clean up cairo handling somewhat

Clean up cairo handling somewhat

I'm building on RHEL6 which doesn't have cairo-gobject.  Now, we
tried to support this in that 'make' would succeed, but 'make check'
would blow up.

This patch allows more of 'make check' to succeed - I just need to
figure out how to make the diff against the -expected.gir work.

What's more important though, is this will allow gjs to also #define
_GI_DISABLE_CAIRO.