Add a CMakeLists.txt file which allows cogs to be compiled via CMake

[jcfreeman2]

As per discussion with Brett, Giovanna and Alessandro on Tuesday morning, this Issue is filed so as to allow cogs to be built within the CMake-based build framework documented at https://github.com/DUNE-DAQ/appfwk/wiki/Compiling-and-running . Technically, it shouldn't need to involve the alteration of any existing files in the repo, but simply the addition of a CMakeLists.txt file and a cogsConfig.cmake.in file (the latter so as to allow cogs to be usable by other projects via CMake's find_package(cogs REQUIRED) command).