make validate work on Windows
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.
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