CMake improvements and addition of Google Test to start converting existing CppUnit tests to it

[amadio]

Opening a pull request to give others a chance to review this before merging. Will merge tomorrow if there are no objections. The CI system is adapted to run the tests already, including the one I converted as an example.

[ccaffy]

All good for me :) Thanks @amadio for this work :)

[simonmichal]

I wondering if we shouldn't guard the following block:

%check
 cd xrootd/build
 %if %{use_cmake3}
 ctest3 --output-on-failure
 %else
 ctest --output-on-failure
 %endif

with %if %{?_with_tests:1}%{!?_with_tests:0}, meaning enforce the check only if we are building with _with_tests flag.

Otherwise looks good :-)

[amadio]

We could do that, but it's not necessary, the way it is now, ctest will just do nothing. You can see what happens here, in the fedora-rpm build.