[cairo] configure script does not find pthreads when librt.a is present

[Ryan Schmidt]

I'm forwarding to you a bug report from a MacPorts user. There is apparently a problem with the cairo configure script where it will incorrectly find a /usr/local/lib/librt.a and try to find symbols in it, which it should be finding in libsystem.dylib instead. Here is our bug report about this:

https://trac.macports.org/ticket/31275#comment:7

This problem is outside my area of expertise so I don't know what to do other than report it to you. If you need more details it might be best to contact the user who reported the ticket.