Make on Linux (Xubuntu) does not work without libminiupnpc8-dev installed.
You mean with
USE_UPNP=- (don't compile support)?
Its set to USE_UPNP=- in line 41 of makefile.unix .
No, it's set by default to
USE_UPNP=0 (compile support, but disable by default) on line 5, and changed from (null) to - on line 41. So eg,
USE_UPNP= will work also (but there is no way to support this syntax with -Qt)