Added command line options, to avoid having to build tests, examples …

[jochemste]

…and gtest.

Using this project as a submodule in CMake, it is not always needed to build the tests and examples. Adding 'option' allows easier enabling/disabling of building these subdirectories. Default is ON, so as not to break anything existing.

Now you can include the project using CMake, like so:

  # Add fff to the build
  set(FFF_BUILD_TEST OFF)
  message("Running on the lib/fff directory")
  add_subdirectory(path/to/fff)