compat/nedmalloc: drop nedmalloc

nedmalloc was initially added in f0ed8226 to fix slowness of standard WinXP
memory allocator. Since WinXP is EOLed, this point is no longer valid.

Tests show that nedmalloc doesn't give much gain on Windows 7+.
Additionally, our current nedmalloc doesn't compile under MinGW-W64.

So, just drop nedmalloc.

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