The current libvpx autoconf check is broken due to the fact that it is attempting to link the library without the appropriate POSIX threads library so it'll fail to link properly.

configure:53841: *** checking feature: VP8 decoder ***
configure:53845: *** for plug-ins: vp8 ***
configure:53885: checking for vpx_codec_version in -lvpx
configure:53920: cc -std=gnu99 -o conftest -O2 -pipe  -I/usr/local/include  -I/usr/local/include/libpng  -I/usr/X11R6/include  -L/
usr/local/lib  -L/usr/X11R6/lib conftest.c -lvpx   >&5
/usr/local/lib/libvpx.so.1.0: warning: sprintf() is often misused, please use snprintf()
/usr/local/lib/libvpx.so.1.0: undefined reference to `pthread_create'
/usr/local/lib/libvpx.so.1.0: undefined reference to `sem_destroy'
/usr/local/lib/libvpx.so.1.0: undefined reference to `sem_wait'
/usr/local/lib/libvpx.so.1.0: undefined reference to `sem_post'
/usr/local/lib/libvpx.so.1.0: undefined reference to `sem_init'
/usr/local/lib/libvpx.so.1.0: undefined reference to `pthread_join'

The -lm -lpthread flags are missing. I moved the OLD_LIBS / LIBS dance to outside
of the use of the AG_GST_CHECK_FEATURE() macro and this has resolved the issue for me but I am not sure if this is the most appropriate means of fixing the autoconf check.