hg head: undefined symbol: tc_libavcodec_mutex

Hi,

I tried to compile the head of the hg repository. It compiles but when I
try to transcode something using -x ffmpeg I get:

[transcode] warning: XXXX/transcode/import_ffmpeg.so: undefined symbol:
tc_libavcodec_mutex
This is clearly in libtcext but I don't understand why it does not seem to
be linked correctly.

Any hints?
  Georg