Makefile clean target doesn't work with GNU make

[sten0]

Hi Paul,

While importing the latest release into Debian, I noted the clean target was broken, but I'm assuming that you don't see it because you're on macOS and are using BSD make.

This is the problem:
https://github.com/rnkn/fountain-mode/blame/f3685ac7235de79cfa9a9c84acbbe8541bcf5e25/Makefile#L60

Please let me know if you'd like to support developers who use GNU (rather than BSD) systems, as well as if you'd prefer the one-line solution that calls find, or something else.

[rnkn]

Thanks for the heads up. I've only considered the Makefile as a convenience thing for me, so portability hasn't been a huge concern, but if you have a fix, by all means!