Fix missing gflags imported target definition in CeresConfig.cmake

- If gflags was built & exported with CMake but glog was not, but both
  were found then as we now make gflags a public dependency of Ceres if
  both it and glog are found, the *name* of the exported gflags CMake
  target (gflags-shared or similar) will appear in CERES_LIBRARIES.
- However, as imported targets are not re-exported, this results in a
  linker error when compiling client code, as the name of the exported
  gflags target is not known to CMake, it assumes it is a library name,
  which it is not.
- Confusingly, if glog was built with CMake, this problem would not
  occur, as in that case glog’s CMake target would bring in gflags’.
- Now we explicitly call find_package(Gflags) in CeresConfig.cmake if
  Ceres was built with gflags as a public dependency (via glog).

Change-Id: I5cc9483a1fae50f4e9e3a8fbba491b645fd45db6