Export: Disallow exported interface includes in src/build tree (#14592).

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.
Merge topic 'install-prefix-in-interface' into next

f6548fe2 Export: Disallow exported interface includes in src/build tree (#14592).