CPack/RPM: Fix weak dep support
RPM 4.11.3 for el7 contains backported support for the RPM weak dep tags. It only supports querying those tags, but rpmbuild can not make use of them. Since CPack relies on rpmbuild --querytags to check for support, this commit switches to rpm --suggests to check for support of weak dependencies.
Tested with RPM 4.11.3 (only supports querying weak dep tags) and RPM 4.14.3 (full rpmbuild support for weak deps)
Fixes: #22350 (closed)