MINGW: compat/poll/poll.c: undef NOGDI

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

Removal -DNOGDI=1 from config.mak.uname has an undesirable effect of
bringing in wingdi.h with weird #define ERROR 0 that conflicts with
internal Git enums. So, just #undef NOGDI in compat/poll/poll.c.

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>