Cmake refactor.

- Split sources from targets. Source files are stored in list variables,
  and targets now use the list vars.
- Surface all build settings control in CMakeLists.txt.
- Remove individual C++11 tests in favor of requiring support
  for --std=c++11.
- Improve organization; stop reopening the same if's.

Change-Id: I1989803fdfd9c032f417a5fc12985671455148bd
3 files changed