Fixes to Valgrind 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.
Can you fix `MaximumMacOSRequirement`  instead at [its definition](https://github.com/Homebrew/linuxbrew/blob/83d0faff5df8b260311494c9befec804075d4b9d/Library/Homebrew/requirements/maximum_macos_requirement.rb#L11)?