This includes automatically adding --build=i386-unknown-mingw32 and
--with-gcc to ./configure, and disabling -j2 (see #1559).

Also: you can now say 

  CPUS=3 ./validate

and the script will pass -jN to make (where N = $CPUS + 1).  In the
future, when we fix #1558, it can pass THREADS=N to the testsuite, too.