Added WSL build support (for developers) (Patreon)

Due to my (dayjob) boss forcing me into a Windows-oriented lifestyle (despite being a big Linux supporter and user), I find myself spending more and more time booted inside Windows rather than Linux.

My big dependence on tools like Visual Studio, PowerShell, and the likes during my dayjob make dual-booting (for the sake of compiling / testing new internal builds) somewhat of a hassle as they interfere with regular work.

Sure, I could run a virtual machine, but I'd rather not unless I absolutely have to due to the rather large overhead in storage space and RAM usage.

Thus, I've taken the time to merge Uldiniad's (and kxzxxx's) WSL build-fixes into our repository (along with a patch of my own to fix a mesa-related build error), enabling myself and other developers to compile our GPD Win / Pocket Android ROM within Windows 10's WSL environment.

Note: The WSL patches only get applied if the patch-script detects a WSL environment, thus regular linux builds will remain unaffected by these changes.