Fix #160

Looking at 'return()' documentation in cmake, i.e.

$ cmake --help-command return
return
------

Return from a file, directory or function.

::

 return()

Returns from a file, directory or function.  When this command is
encountered in an included file (via include() or find_package()), it
causes processing of the current file to stop and control is returned
to the including file.  If it is encountered in a file which is not
included by another file, e.g.  a CMakeLists.txt, control is returned
to the parent directory if there is one.  If return is called in a
function, control is returned to the caller of the function.  Note
that a macro is not a function and does not handle return like a
function does.

call to 'return()' in a macro explains the error, it would return after each call
and files would never get installed.

instead of using a macro, we now use a function.

Co-Authored-By: Stefan Schlager <zarquon42@gmail.com>
Merge pull request #162 from statismo/install_testing_on

Fix #160