Add a cmake build system.

For those of us that don't like rebuilding the whole damn thing every
time.
Next layer of cmake
Add a cmake build system.

For those of us that don't like rebuilding the whole damn thing every
time.

Will add support for tests with lit in a later patch.

This does not yet support building the unwinder in src/Unwind.
Some updates to cmake system.

RTTI isn't optional, it's required.
Same for C++11.
Default to static library, with dynamic library as an option.