build: Fix pycairo detection "script" for Windows

Hi,

From the commit message:

For the source path that we obtained from Meson, prefix that path with 'r' so that the pycairo detection test script does not choke on Windows-style paths, which will fix detecting pycairo without having to resort to pkg-config to find pycairo, which may well not be possible as pycairo may have been installed via PIP without the pkg-config files.

With blessings, thank you!