ecm_add_app_icon should be a noop outside WIN32 and APPLE platforms, so the if()... should not be needed. Did you hit any issue that led to this change?

Actually yes. ECM on Linux complains about missing option "OUTFILE_BASENAME" when the ECM version is < 5.49.
Is there any way to check the ECM version?

But shouldn't ECM complain regardless of the operating system? Or do you have different requirements for ECM under different operating systems? Those should be kept aligned: if you require that option, you are basically requiring ECM 5.49 everywhere.

This problem should be fixed with the latest commit 98a74667bddbfa1ad720d92249b91831dbfe6581