Facilitate the addition of warnings into the build system

The proliferation of compile warnings needs to migrate from BUILD/* scripts to mainstream CMake maintainer mode that is accessible to everyone.

The current set of warnings there provides an error on CMAKE_BUILD_TYPE=Debug but a warning on CMAKE_BUILD_TYPE=RelWithDebInfo.

We need a experimental region of warnings to introduce to the codebase, then clean up the errors across all branches, and then make into the strong -Werror mode of Debug.

from marko, -Wreorder and -Weffc++ are good options, though the second produced an unrealistic number of RocksDB warnings (which are expected to go away after a submodule update.