Slightly rework the comments and logic for default Clang/GCC.

This is because the previous version was very obscure about the fact
that despite having Clang "on by default" for architectures such as powerpc, it
does not actually build due to the GCC it uses not having C++11 support.
Using an external compiler that supports C++11 does allow this to work.
This whole block should be rethought more given "on by default" is not
really default without extra work which could actually be surprising for
why Clang is showing up when using a newer GCC.

Sponsored by: EMC / Isilon Storage Division