[cairo] Making configure error message more clear

[Arjen Nienhuis]

> Below is an exchange I recently had with a new user trying to use cairo
> who got this error and followed its advice, (browsed to the URL, found
> pixman, downloaded it, installed it to the default /usr/local/). But the
> user still got the same message, and never knew to adjust the
> PKG_CONFIG_PATH environment variable.

Why isn't PKG_CONFIG_PATH set to $PREFIX/lib/pkgconfig by default?