toolchain: temporary build fix for MinGW-w64
toolchain: temporary build fix for MinGW-w64

Build from a known-working commit since both Git master and the
current stable release result in build errors later on (FFmpeg
and fontconfig, respectively). The FFmpeg build failure with
master is most likely FFmpeg’s fault, as it seemingly uses
pthread functions without requiring pthreads from the toolchain.

chhe/mingw-w64-cmake,lachs0r/mingw-w64-cmake