Use the correct binary name for GNU diff on Solaris.

People still use Solaris! On Solaris, GNU diff is called 'gdiff'. Some of our
code (particularly in the perforce client) handled this, but the check to make
sure that we were using GNU diff didn't.

Testing done:
I haven't had access to a Solaris machine in over a decade (eep), so I couldn't
test that part, but I ran the unit tests to make sure I didn't fudge up
anything else.

Fixes bug 3334.

Reviewed at https://reviews.reviewboard.org/r/5793/