Niakr1s

1. Put app.ico in directory.

2. Create app.rc in same directory with one line:
IDI_ICON1               ICON        DISCARDABLE            "app.ico"

3. Run command (Warning: it's app.o, not app.res, how it is mentioned in other manuals!)
windres app.rc -o app.o

4. add_executable(app
  ...