Revert "mingw32-git: Set the correct sysconfdir (for "git config --system")"

This reverts commit bd5dcc5. Do not set sysconfdir manually at all to
avoid an absolute path being hard-coded into the git executable. Instead,
let git figure out the relative location at runtime.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
mingw32-git: Get rid of the last hard-coded compile-time path

Use the DESTDIR approach for installing to get rid of all references to
the compile-time install path and the fragile prefix variable stuff.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>