MakeMaker.pm: Search for libssh2 includes and libs in /usr/local (on FreeBSD)

This "should" fix #49 (Untested). I did a slight more generic approach, as similar problems may start showing up on other platforms, notably Solaris.

It would be nice to make this even more configurable, I'll see if I can dream something up (that also works with Windows and MSVC).