Make sure for the mysys project that __NT__ is defined in *nt solution
configurations (but not in other configurations).


VC++Files/mysql.sln:
  Define __NT__ in mysys for *nt configurations.
VC++Files/mysys/mysys.vcproj:
  Add configurations with __NT__ defined.
mysql-test/mysql-test-run.pl:
  Also allow testing a "Max nt" build.