Clean up all the different possible CMake options for specifying the ABI include paths into one CMake option named LIBCXX_CXX_ABI_INCLUDE_PATHS.
The documentation has been updated to reflect this change.
For the next week I have added explicit errors if any of the old flags is used. These errors inform users of the change and the new option to use.
Before committing the change I will announce this change on cfe-dev.