Warn instead of error if ecm_install_icons finds no icons.
Review Request #125931 - Created Nov. 3, 2015 and submitted
The V1 syntax of ecm_install_icons searched for icons by globbing files with a particular naming pattern. If there were no such icons, this used to do nothing, but silently. Commit fb7b8eea7d accidentally made this an error. More sensible would be to make it a warning. BUG: 354610
Extended unit test, which passes, and ksirk now successfully configures.