In the makefile, add the MYICON.ICO file to the list of files on which ICON.RES is dependent. The relevant lines in the makefile should look like this:
icon.res: icon.rc icon.h myicon.ico
rc /r icon.rc
This change ensures that, if the MYICON.ICO file changes, ICON.RC will be recompiled to form a new ICON.RES file.
No other changes are required.