Added support for Win32, and cross-compiling with MXE

The cfg-all script now checks for MXE (http://mxe.cc/) and if
found, sets up an extra build directory for cross-compiling to
32 bit Win32 using that. You may need to pass the part of your
MXE install to cfg-all, and/or modify it as needed.

Fixed some Win32 portability issues related to SDL (strndup() and
varying ideas about what the correct prototype for main() is...),
and also fixed the test programs that are NOT using SDL. (There is
no portable sleep() function.)