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.
Allow directories in the source tree or build tree only if the
install tree is a subdirectory of the source tree or build tree,
as appropriate.
Re-use the test files in the RunCMake.include_directories test
to run in multiple scenarios. Bump the required CMake version
in the test to 3.0 to ensure that the new policy warnings are
emitted correctly.