Log Message:
* Fix libavcodec library name detection, so we don't later try to
  dlopen("-lavcodec.51").
* Disable h263ffmpeg plugin as ekiga doesn't use it
* Pass LDFLAGS to plugins at linking time, so they get RPATH
* Use RTLD_NEXT on NetBSD in an attempt to find symbols in NEEDED
  libraries of the one we actually loaded (libavcodec).