Show the git version in the log + other places

Replaces PPSSPP_VERSION_STR.  I renamed it to make sure I got every occurrence.

Not sure where to put git-version.cmake, or if I should've put git-version.cpp elsewhere.  Just on the root now...

Tested on Linux/Qt, Mac, and Windows.  I'm not super confident with cmake but it seems to work fine, anyway.

-[Unknown]