run-make-fulldeps/hotplug_codegen_backend is too presumptuous in checking for its dependencies

Namely, it has 

```
	/bin/echo || exit 0 # This test requires /bin/echo to exist
```

however, it is not guaranteed that this binary will be exactly at `/bin/echo` on all systems. For example it isn’t on mine:

```fish
which echo # /run/current-system/sw/bin/echo
ls /bin/ | grep echo; or echo "echo is not in /bin/" # echo is not in /bin/
```