Restore header installation on Mac to always install to $includedir

We haven't fully agreed on whether headers should be in $includedir or
not for builds with frameworks, since frameworks carry their headers
inside. However, this change came too late for Qt 5.1 for us to assess
the potential impacts -- it's known to break at least the cmake files we
ship.

So restore the build to the way it was.

This is a partial revert of 6d61dfdbb74a2055438b999c6962f89cc3388eea.
Partial because the old behavior was partially broken: it did not
install headers for release builds. Now we always install (and in
debug-and-release mode, we might do it twice).

Task-number: QTBUG-32134
Change-Id: Ib84879c5a148d3717d16a7a90b2f5735fb5d80be
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>