[Bug]: CMake: juce_add_gui_app(ICON_*) does not work with .icns or .ico

[ofarrill]

Detailed steps on how to reproduce the bug

CMake generates either an empty icon when used with .icns under macOS; or claims not find the icon when used with .ico under Windows.

What is the expected behaviour?

Cmake accepts the standard OS icons. (Nothing has to be converted in this case).

Operating systems

Windows, macOS

Testing on the develop branch

The bug is present on the develop branch