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>
ALL PREVIOUS REWORKED
MINGW: config.mak.uname: drop USE_NED_ALLOCATOR

nedalloc was initially added in f0ed82 to fix slowness of standard WinXP
memory allocator. Since WinXP is EOLed, this point is no longer valid.

The actual reason behind this commit is incompatibility of malloc.c.h
with MinGW-W64 headers. Alternative solution implies updating nedalloc
to something newer.

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>