MINGW: install into /mingw/bin/ by default

In msysGit, we collapsed all bin/ directories into one, so it made sense
to install Git into /bin/ right away.

With the new Git for Windows SDK, we want to clean that up a little and be
more standards-compliant. In particular, we will use most of MinGW as-is,
and provide Git as a proper MinGW package. So let's install the files into
the correct place.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #8 from dscho/mingw-prefix

MINGW: install into /mingw/bin/ by default