Build a set of Windows 32-bit 'legacy' binaries with VS2003.

Build a set of Windows 32-bit 'legacy' binaries with VS2003.

The downside of moving to VS2015 is that its output won't run on very
old versions of Windows. It's not yet clear whether anyone still cares
about things before, say, Win2000 or WinXP, but since my build
environment still _has_ VS2003 available, it's easy enough to build
the extra set of binaries anyway just in case. (At least for now.)

The new binaries live in a build output directory 'w32old'. As with
w64, there is no installer for them; but unlike w64, I don't intend to
add one.