threading: [partial] Working NetRadiant and q3map2 on MSYS2 (both x86, x86_64)

threading: [partial] Working NetRadiant and q3map2 on MSYS2 (both x86, x86_64)

this code is meant to fix q3map2 threading on MSYS2

partial commit from original by Aleksandr “void995” Palamar:

- c1c3c66ec062438e1a99266751122557517d708d

initially named:

> Working NetRadiant and q3map2 on MSYS2 (both x86, x86_64),
> fixed problems with out of bounds and got rid of unneeded
> warning messages. Optimisations for MSYS2 were disabled as
> they are causing crashes. Added specific MSYS2 shell scripts
> for installing DLLs and GTK themes.

see merge request !18

https://gitlab.com/xonotic/netradiant/merge_requests/18