Try different way

code/unix/Makefile

@@ -1767,7 +1767,8 @@ $(B)/missionpack/qcommon/%.asm: $(CMDIR)/%.c
 copyfiles: build_release
 	@if [ ! -d $(COPYDIR)/baseq3 ]; then echo "You need to set COPYDIR to where you installed Quake III!"; false; fi
 	$(INSTALL) -s -m 0755 $(BR)/$(PLATFORM)quake3$(BINEXT) $(COPYDIR)/quake3
-	ifneq ($(PLATFORM),mingw32)
+	@if [ -f $(BR)/$(PLATFORM)q3ded$(BINEXT) ]
+	then
 		$(INSTALL) -s -m 0755 $(BR)/$(PLATFORM)q3ded$(BINEXT) $(COPYDIR)/q3ded$(BINEXT)
 	fi
 	-$(MKDIR) -p -m 0755 $(COPYDIR)/baseq3