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.
We can do this check only if the TargetName is non-empty, which means
that we're evaluating INTERFACE_INCLUDE_DIRECTORIES from a linked
dependency which was set using target_link_libraries.
It is possible to have relative paths in INCLUDE_DIRECTORIES already
in CMake 2.8.10.2, so that part will require a policy to fix.