Use portable way of getting modified time

sslh 1.16 compiles fine on OS X. There's just one little issue: `stat -c` is GNU-only, so the version detection fails; this results in a reported version of "zip-".

This is not my area of expertise, but I grabbed a more portable equivalent of `stat -c "%y"` from the following link, and it works as expected for me.
http://ginini.com/techblog/displaying-the-modification-date-of-a-file/

Maybe there's an even better solution?