modplug's sndfile.h conflicts with libsndfile's sndfile.h, so
we'll access it directly using modplug/sndfile.h. Fixes#573849
Signed-off-by: David Schleef <ds@schleef.org>
Always build the modplug plugin against the system modplug library
and remove our own copy. Using the system version has advantages
if security issues or other critical bugs are found in libmodplug
and our own copy wasn't really maintained anyway.
Also our copy only contained some patches to use GLib types and functions.
Fixes bug #568837.
