[cairo] OS X backend

[Carl Worth]

On Dec 5, Owen Taylor wrote:
 > It's a fairly good assumption that if someone sets ACLOCAL_FLAGS,
 > then they will set it correctly.

Yup. Good, but not perfect, (as demonstrated above). I don't see an
easy way to do better. And this only comes up when building from CVS,
so I think we're OK.

 > I've Cc'ed Shawn Amundson who wrote the test, but I think it should
 > be fine to steal in any case ... it's not doing anything 
 > non-obvious.

Thanks. We grabbed this sometime ago -- that's nearly identical to
what is already in cairo's autogen.sh. Thanks Shawn!

-Carl