[Mesa-dev] Fix the build on Windows

[Mathieu Bridon]

This is follow up to my recent patches which made the Python build
scripts compatible with both Python 2 and 3, then moved the Meson build
system to using Python 3 for them.

Unfortunately, one thing I hadn't tested was running them on Windows,
where the changes broke the build.

See the exchange with Jose for the details:

https://lists.freedesktop.org/archives/mesa-dev/2018-August/202847.html

These 2 patches have already been tested by Jose, who confirms they fix
the issue he countered.