"USE_UPNP=-" is needed to remove UPnP support.

doc/build-unix.txt

@@ -33,7 +33,7 @@ Dependencies
 miniupnpc may be used for UPnP port mapping.  It can be downloaded from
 http://miniupnp.tuxfamily.org/files/.  UPnP support is compiled in and
 turned off by default.  Set USE_UPNP to a different value to control this:
- USE_UPNP=     No UPnP support - miniupnp not required
+ USE_UPNP=-    No UPnP support - miniupnp not required
  USE_UPNP=0    (the default) UPnP support turned off by default at runtime
  USE_UPNP=1    UPnP support turned on by default at runtime