support third party library configuration in qmake (#611)

support third party library configuration in qmake (#611)

* add qmake options to specify libraries.

* fix no libusb along lines of #610

* correct inclusion of shapelib headers.

* more closely match original includes

instead of trying to use HAVE_*, ENABLED_* *_INHIBITED and
-I options in a consistent manner.

We can revisit this after the demise of configure.

* add qmake message when libusb is disabled.