Support building this as a DLL under Cygwin.
Noone but me cares, but Guido said to go ahead and fix it if it bothered me.
Wasn't built on Windows; not in config.c either. Module init function missing DL_EXPORT magic. test_xreadline output file obviously wrong (started w/ "test_xrl"). test program very unclear about what was expected.
changes for safety and tuning.