Enabled via configure --ccache, or CONFIG += ccache in 3rd party
projects.

Ensures that we use the right sloppiness and other ccache options
during compilation.

Task-number: QTBUG-31034
Change-Id: I696b3d3f0398873a29b93d1bc2b4d4e06ef23dc9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>