Remove unnecessary language initializations. Change the main
CMakeLists.txt project() call to specify NONE. Use enable_language(CXX)
instead of project() in cases that need it to avoid enabling C too.
It is considered an error if the INTERFACE_INCLUDE_DIRECTORIES contains
a directory which does not exist, which indicates a programmer error
by the upstream, or a packaging error.
One of the RunCMake.CompatibleInterface tests also needs to be updated
due to this change. Non-existant includes were used in the test, but
are not needed.