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.
I'm not really a ruby programmer in any sense, I could look into it but it probably won't be for a few weeks.  You could toss the if OS.mac? condition and everything will still compile, it will just complain a little bit about OS X versions.