dev-qt/qtbase: tentatively enable unity build in live

Better known as jumbo-build from chromium/qtwebengine.

This affects most dev-qt/* revdeps too, similarly to having it set in
qt6-build.eclass. Build time reduction seems to be around 30% which
can be significant for the bigger packages and slower machines.

Not adding IUSE=jumbo-build given there should be little reason for
a user to turn this off. It *can* make some sense on chromium due to
the increase in memory usage, but that is rather insignificant on
other packages and this has reasonable size defaults. Can still
MYCMAKEARGS=-DQT_UNITY_BUILD=OFF if really must.

Could potentially introduce build failures on bumps or some untested
USE combinations depending on how often/well Qt upstream tests this
option. Can temporarily disable it per-package when needed (e.g.
qttools is known failing with USE=designer for one so far),
not really worth worrying about getting these actually fixed asap
unless it is qtbase given then we'd lose it for the whole stack
unless move the option to qt6-build.eclass.

Signed-off-by: Ionen Wolkens <ionen@gentoo.org>

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