trivial: style changes in top-level CMakeLists.txt

Changing comments and whitespace to reduce the diff with the protobuf branch.

If you want justifications for such minor changes, they are:

* Keep all the Boost changes clumped together, under one heading. Makes things easier to read when there's another dependency.
* The middle change keeps the compiler options under 80 chars (not that I am a fan of that limit).
* No justification for the alphabetization.