data is not installed by gmod by default, try to copy the bsp to prevent

invalidating the nav file

Makefile

@@ -175,7 +175,8 @@ maps/%.bsp: %.bsp
 	rm -i "$(MOD_DIR)/mapsrc/setphys.vmf"
 
 %.nav: %.bsp
-	ln -f "$^" "$(MOD_DIR)/maps/navgen.bsp"
+	cp -v "$^" "$(MOD_DIR)/maps/navgen.bsp"
+	mkdir -p "$(MOD_DIR)/data/"
 	cp -v lua/nav_generate.txt "$(MOD_DIR)/data/runme.txt"
 	ln -f lua/nav_generate.lua "$(MOD_DIR)/lua/autorun/runme.lua"
 	$(SHLEX_BIN) $(SHLEX_FLAGS) $(SHLEX_HL2LOG_FLAGS) \