cmake: Add in UHDConfig*.cmake files and install them into CMAKE_MODULES_DIR if provided (default value is "lib${LIB_SUFFIX}/cmake/").  Tested to work with GR's new FindUHD.cmake (as a daisy-chained "find_package(UHD ...)") as well as directly.
cmake: set ENV variables to tell GR's FindUHD.cmake that these files were actually used; UHDConfig*.cmake are not always allowed to use CACHE variables, thus the use of ENV.
uhd: Added an example for CMake usage