CMake: Make QUIET argument work

QUIET was added 16e2543dcea1ad25d238424b8c78c3d17c256c9d as a
synonym to SILENT, with the indicatoin that SILENT might be dropped
at one point in the future. Anyhow, already the original commit
didn't touch all places where SILENT was checked, and
commit 89ee6f7c6ca9e08184e8cf7176c2023beb714ac8 did finally remove
the only check where QUIET was actually checked, making it a NOOP.

Follow up with the intention of the original commit, and make
QUIET and SILENT truely synonyms.

Pick-to: 6.12
Change-Id: I60f83a70521b5fa43a7bf7f436e0517b4f93e6cf
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>