mingw32-git: Use default gitconfig file

This is needed to provide the path to the curl-bundle.crt to git
using the system config file.

Turned out to be way more complilcated to set because the way mgwport
works. The path prefixes are different for the comilation and the
installation phase, but there git's makefile logic inteferes and
you end up with a big mess.

The solution is to remove the GIT-PREFIX, GIT-CFLAGS, GIT-LDFLAGS checks
and recompile all config.c and attr.c as both require the proper path
to /mingw/etc be set.

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>