valgrind 3.10.1: Fixes for Linuxbrew

Conditioned the flag for `--build=amd64-darwin` on the host being OS X, otherwise the formula fails.  A few  OS X version checks were also conditioned on the OS being OS X, this is mainly for aesthetic reasons.
Okay. I'll be on vacation, but should still find a few minutes to merge a PR if you're pushing commits on Christmas Eve.