Build warning for threads

[stonier]

From this github action for kobuki.

  The package name passed to `find_package_handle_standard_args` (Threads)
  does not match the name of the calling package (ecl_threads).  This can
  lead to problems in calling code that expects `find_package` result
  variables (e.g., `_FOUND`) to follow a certain pattern.

occuring whenever find_package(ecl_threads) is called. Can't reproduce locally - the github action is using a custom installed 3.17 version of cmake in /usr/local, whereas the official release for ubuntu 20.04 is v3.16.

Not urgent (seems to be just providing guidance to adhere to capitalisation in names more strictly), wait till I have a v3.17+ available so I can reproduce locally.