Obtain the location of the include files by querying
the qmake variable QT_INSTALL_HEADERS by default, which
can be overridden by the command line option.

Task-number: QTBUG-45662
Change-Id: I03a781e9b98f5e2980dbaef86eedd05aec0103ce
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>