Suppress RANS compiler warnings for clang

This should clean up the compiler warnings that occur with the rans tests when compiled with Clang.

Based on: https://stackoverflow.com/questions/14065844/how-to-suppress-the-deprecation-warning-dispatch-get-current-queue-is-depreca and !2888 (merged)

Not sure if there is a neater way to do this. Macros are always pretty ugly, and these are particularly gnarly looking.