ITS#10007 Fix running on Cygwin/MSYS2

Christopher Ngrequested to merge
facboy/openldap:its-10007 into mdb.RE/0.9

This commit allows lmdb to run correctly on Cygwin/MSYS2 when it is compiled with make CPPFLAGS="-DMDB_USE_POSIX_SEM=1".

Signed-off-by: Christopher Ng facboy@gmail.com

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