MINGW: config.mak.uname: drop -DNOGDI

On MinGW-W64, MsgWaitForMultipleObjects is guarded with #ifndef NOGDI.

Unfortunately, including wingdi.h brings in #define ERROR 0 which
conflicts with several Git internal enums. So, #undef ERROR.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>
MINGW: config.mak.uname: add explicit way to request MinGW-build

When crosscompiling, one cannot rely on `uname` from host system.
Thus, add an option to use `make MINGW=1` for building MinGW build
from non-MinGW host (Linux, for example).

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>