build: Use a fallback path to generate mex-version.h
build: Use a fallback path to generate mex-version.h

When compiling from a git repository. This allows people to use raw
snapshots of the git tree (ie without the .git directory.

Fixes: https://github.com/media-explorer/media-explorer/issues/60

media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer,media-explorer/media-explorer