Add missing dependency on libavutil if found

[pH5]

src/log.c uses av_log_set_level() and av_log_set_callback() from libavutil if HAVE_LIBAVUTIL is set. This causes a build failure if libavutils development packages are installed, but the other dependencies for h264 are not.

[any1]

There's really no reason to include avutil if h264 is disabled. Better just move config.set('HAVE_LIBAVUTIL', true) to the same place where ENABLE_OPEN_H264 is set for now.

[pH5]

Moved as suggested.

[any1]

Thanks!