fix non-git non-shadow non-prefix non-qtbase builds

these builds usually assume all headers in the qtbase build (== source
== install) dir, so the path for adding our pre-generated per-module
include paths needs to be triggered explicitly.

Change-Id: I57ec441d58cdf8186907ee6c36dce08daa206c49
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
don't add absent private include paths to non-git builds

all private headers are created by syncqt (and are thus in the source
dir), so we can simply override the normal (build dir based) paths
instead of extending them.

Change-Id: I9c1f3344c401b481b3f3d2295515f1aabffaa9a0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>