irrlicht: fix linking.

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.

Closes #26881.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>