If libupnp is installed in a non-standard location we must rely on the
include path provided by $(pkg-config --cflags libupnp). Relative to the
path given from that command no prefix must be used to find the respective
files.
Eliminate class LibUPnP and move the code to ClientInit.cxx. Its
initialization function can be called multiple times, but
UpnpRegisterClient() is called at most once.
