Add support for Meson build system
Initial support for Meson build system, plus fixes for MSVC and Windows build in general
Tests don't all work on Windows yet, but that's the same for the Autotools build (doesn't even manage build tests on Windows), let's investigate and fix that separately.
Build has been tested on Windows with MingW, VS2015 and VS2017 in Cerbero, and on Linux.
On Linux the test suite succeeds/fails in the same manner as it does with Autotools (
test-fullmode-with-stun fails to gather candidates).
I have another commit that bumps the Meson requirement to 0.49 but I'm holding that back for now because the CI image only has 0.48.2 and it's only for g-i with static libs anyway.
I'm sure some more fixes here and there will be needed, but this should give us a good base going forward.