Fix solid build against libudev < 167
Review Request #113404 - Created Oct. 23, 2013 and submitted
Afiestas suggested improving on https://projects.kde.org/projects/kde/kdelibs/repository/revisions/a8d3d3321522d35c2852abb02a475c6b43be5cfe To fix build against libudev < 167 , instead of disabling udev solid backend altogether, only omit the piece that needs the newer api. Gratuitously added pkgconfig HINTS too, but can omit that part if it is considered unwanted.
Builds now succeed on rhel6/centos6 hosts that failed previously.