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.
The following one-line patch should work. If you can test it out, I'll merge the PR. No rush though. It can wait until the new year.
```diff
-  satisfy { MacOS.version <= @version }
+  satisfy { !OS.mac? || MacOS.version <= @version }
```