Always ensure that the hardcoded paths are found

Practice has shown that quite many sets XDG dirs for various purposes
and that spoils qtchooser finding the distro provided Qt without adding
like /usr to the XDG dirs variable which kind of kills performance.

Provide a dedicated variable to disable a global installation.

Change-Id: I58953e3b2b3cf43cb67d4c367312e83cfa9ad2b3
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>