fix pty related tests issues on Windows * lib/grantpt.c (grantpt): grantpt.c seems to be never used on Windows, however it's still present in tests/ subdirectory of the final project. Therefore avoid it to pass `make check`. * tests/test-openpty.c (main): Windows has no PTYs and gnulib's openpty() will just return -ENOSYS, so avoid this non applicable test allowing the build to proceed.