People tend to "turn off debug and release builds" by just not building
one of the variants. For example, Qt's own rcc is built in release only,
however it is configured for debug_and_release with the same TARGET for
both.
Let qmake complain about conflicting TARGETs only we're about to build
all of those conflicting targets, i.e. if build_all is set.
Change-Id: I0448bf5cb421e2d801d3cc30e0d80353fba0d999
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
5ab8efd66a