ci: reeanble compilation test with clang -O2, disable -Wmaybe-uninitialized for old gcc

ci: reeanble compilation test with clang -O2, disable -Wmaybe-uninitialized for old gcc

In CI we get spurious failures about unitialized variables with gcc
versions older then (depending on the case) 12, 13, or 14. Let's only
try to do this check with newer gcc which returns more useful results.
At the same time, do compile with both gcc and clang at -O2, just
disable the warning.

The old logic seems to have been confused. We compile with -Wall, at
least in some cases, which includes -Wmaybe-unitialized. So if we
_don't_ want it, we need to explicitly disable it.