tweaking of gaudi_install_headers
requested to merge pseyfert/Gaudi:GAUDI-1231-tweak-test-build-of-all-headers into GAUDI-1231.add-test-build-of-all-headers
- avoid creating test targets if they already exist (multiple calls of gaudi_install_headers by users)
→ this one is not an issue in the master branch of gaudi, because it only affects the test targets
- add warning if user calls gaudi_install_headers twice
→ this one can actually be factored out and go to master
- respect a package's compilation dependency for test_headers_build_* target (relevant for #include )