irrlicht fix linking on 10.8

The irrlicht formula is being built using xcodebuild with
an xcodeproj file that has the base SDK specified as 10.9.
There are missing symbols when trying to link against this
on 10.8, so this commit specifies the 10.8 sdk when
mountain lion is the system os.

To reproduce the linking error:
~~~
$ brew install https://raw.github.com/osrf/homebrew-simulation/master/chrono-engine.rb --with-irrlicht --HEAD --verbose
…
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
…
Undefined symbols for architecture x86_64:
  "___sincos_stret", referenced from:
...
~~~

I noticed from [this comment](https://github.com/hashdist/hashstack/issues/91#issuecomment-31491695) that the missing `___sincos_stret` symbols are in the 10.9 sdk, but not in 10.8.