fix UPNP_DEFINES in configure.in to get UPnP logging and xbmc specific HTTP User-Agent working

Finally found the time to look into why UPnP logging and the xbmc specific HTTP User-Agent header weren't working on linux and I found that the definition of UPNP_DEFINES in configure.in was wrong and contained unescaped double-quotes inside a double-quoted string which results in UPNP_DEFINES being empty.