`.pc` files with Windows paths

[tkralphs]

I would like to think about whether we can/should produce .pc files with Windows paths in them when building with Visual Studio in MSys. This is because if we have such .pc files, it will make building Python extensions in Windows a lot easier, as we'll be able to rely on pkg-config to find libraries, which we cannot currently do. I believe this should be possible, we just need to convert paths with cygpath when building the .pc files. I'll think a bit about how to do it.

[tkralphs]

I just became aware that the Windows version of pkg-config has an option --msvc-syntax that converts POSIX paths to Windows paths and also converts the flags from gcc-style to MSVC-style, so this may obviate the need for this. I haven't had a chance to try it yet, though.