REG: Configure now defaults to -system-harfbuzz when available on system

Before the changes to the configure system, we would default to -qt-harfbuzz and only test for system compliance if -system-harfbuzz was explicitly requested.

But in the new system, we default to -system-harfbuzz on all machines which have it installed, making CI fail because of outdated Harfbuzz-installations in RHEL and our binary packages unusable for Linux users who have not installed this library.