As reported in https://github.com/NetHack/NetHack/issues/391
if make was invoked with -j, makedefs instances could end up running in
parallel and could trample on each other's grep.tmp tempory files.
Default to using mkstemp(); allow a port runtime library implementation
that lacks mkstemp() to define HAS_NO_MKSTEMP to revert to the old behaviour.
Provide a work-alike mkstemp() implementation for windows Visual Studio build
in mdlib.c so there is no requirement to define HAS_NO_MKSTEMP there.
Fixes#391
1a0ee44760