Fixup cmake

It looks like there are two minor issues with the CMake scripts:

1. We do not really use anything from CTest.cmake and can avoid including it.
2. The list_sources function fails when there is an interface library (we currently don't have any though) in the project and CMake 3.18 or older is used:

   CMake Error at cmake/list_sources.cmake:42 (get_target_property):
     INTERFACE_LIBRARY targets may only have whitelisted properties.  The
     property "SOURCES" is not allowed.

3. We can pass more than one source file to the add_icon_c_test macro so the second variable is renamed to file_names (by the way, why does the macro have icon in its name?)
4. We use C++14 features in the tests (see !28 (comment 182104)), so we now require C++14 instead of C++11.