compat/nedmalloc/malloc.c.h: fix compilation under MinGW-W64

1. Unlike MinGW, MinGW-W64 already provides _ReadWriteBarrier macro,
   so don't try to redefine it.

2. MinGW-W64 has a strange definition FORCEINLINE as
   extern __inline__ __attribute__((__always_inline__,__gnu_inline__))

   'extern' doesn't work together with 'static', so #undef MinGW-W64
   version of FORCEINLINE.

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>