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.
libupnp 1.6.24 added a few badly designed macros which break the MPD
build:
8177a4195a/
To work around this, we disable our emulation functions (from
714011c81e) on this libupnp version.
Closes#163
