Tor master is broken with mingw-w64

It seems tor master is broken if one is using mingw-w64 for cross-compilation:

https://jenkins.torproject.org/job/tor-ci-mingwcross/791/.

We've hit that with our nightly build setup, too:

../src/common/util.c: In function 'check_private_dir':
../src/common/util.c:2085:45: error: 'O_NOFOLLOW' undeclared (first use in this function)
   fd = open(sandbox_intern_string(dirname), O_NOFOLLOW);
                                             ^
../src/common/util.c:2085:45: note: each undeclared identifier is reported only once for each function it appears in
  CC     src/ext/csiphash.o
  CC     src/common/compat_winthreads.o
  CC     src/common/aes.o
  CC     src/common/crypto.o
make[1]: *** [src/common/util.o] Error 1