GNUmakefile: Allow cross-build for Windows

[oech3]

Duplicated build management is nightmare. We should drop make support...

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/shuf/shuf-reservoir (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/sort/sort-stale-thread-mem (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/pr/bounded-memory is no longer failing!
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.

[cakebaker]

Thanks!