Branched for 5.0-alpha12

[starkos]

Just an FYI that I've spun off a new branch for the 5.0-alpha 10 release:

    https://github.com/premake/premake-core/tree/release-5.0-alpha12

I've already run most of my sanity checks and fixed a number of compatibility issues, so I should be able to finalize the release quite soon. Extra testing of these release branches is always appreciated!

Thanks!

-st.