Cleanup CMake build system using find_package() commands for required libraries.

Changes in this patch:
  * Instead of looking for binaries, we look for the entire package using the CMake `find_package()` command.  This will find libraries and include files.
  * We skip the failure detection, because the `REQUIRED` parameter to `find_package()` fall if the command cannot be found.

Tested on Debian x64 Sid, using CMake 2.8.11.2.  Properly fails when the packages are not completely found (i.e., when the header files cannot be found, even if the library binaries can).  I do not have CMake 2.6 on hand to test this patch, but since it is the minimum required version, this patch should be tested with it before merging.