I don't have a windows machine handy at the moment, so it's hard for
me to figure out the right fix. :-( But it looks like windows defines
_strtoui64 in its stdlib.h, rather than in windows.h. So #including
windows.h won't do any good. And msys is probably giving you its own
stdlib.h, rather than the windows one.
In that case, msys should define strtoll itself. Try this: remove the
"#ifdef HAVE_WINDOWS_H" stuff entirely, and see if the code then
compiles under msys/configure. If so, I think we'll just add a test
for _strtoi64 in configure, and solve the problem that way.