Adapt macOS snowflake compilation to new toolchain
We need to adapt our macOS toolchain due to new Firefox requirements when switching to ESR 60 (the new one will use a different cctools, clang 3.9.1 and the SDK 10.11). This means the snowflake and related projects need to get updated, too.
I'll disable building and shipping snowflake until this bug is fixed (alas, it's not a top prio right now for us) in order to get macOS nightlies based on ESR 60 going as fast as possible.