mingw-w64-git: unify the Makefile targets
mingw-w64-git: unify the Makefile targets

We currently write out two custom files containing Makefile rules.

That is unnecessary. Let's just unify them into a single file:
mingw-w64-git.mak

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

git-for-windows/MINGW-packages,dscho/MINGW-packages,dscho/MINGW-packages,git-for-windows/MINGW-packages,git-for-windows/MINGW-packages,dscho/MINGW-packages,dscho/MINGW-packages,dscho/MINGW-packages