configure: use a macro to check for pthread

Replace the hardcoded -lpthread in most of the places with $PTHREAD_LIBS. For
openh264 also add $PTHREAD_LIBS to OPENH264_LIBS until upstream ships a .pc
file.
Automatic update of common submodule

From a8c8939 to 84d06cd
vtenc: Fix locking

The object lock only protects the session, as we modify
the session from other threads when the bitrate property
is changed. Don't hold it much longer than for session
related things.

And we need to release the video decoder stream lock before
enqueueing a frames. It might wait for our callback to dequeue
a frame from another thread, which will then take the stream
lock too and deadlock.
gl/cocoa: Fix compiler warning

gstglwindow_cocoa.m: In function '-[GstGLNSView drawRect:]':
gstglwindow_cocoa.m:555: warning: 'GstGLNSView' may not respond to '-reshape'
gstglwindow_cocoa.m:555: warning: (Messages without a matching method signature
gstglwindow_cocoa.m:555: warning: will be assumed to return 'id' and accept
gstglwindow_cocoa.m:555: warning: '...' as arguments.)
configure: simplify openh264 check

Of course PTHREADS_CFLAGS is not used anywhere now
for the check, but that was the case before as well.
configure: improve openh264 check

The openh264 API is still unstable, so check for
structure member that we need to prevent building
against an older incompatible version.
rawparse: Don't try to retrieve 0 byte buffers from the adapter in multi-frame mode
rawparse: Check for 0-size after adjusting the size for the frame size

If we don't have a complete raw audio frame we would otherwise still
ask for a 0 sized buffer from the adapter.
intervideo: Properly pass through caps from the sink to the source

Otherwise a magic capsfilter after the source is required with
exactly the same caps as the input.

This would've failed before with invalid buffer sizes:
gst-launch-1.0 videotestsrc ! intervideosink  intervideosrc ! "video/x-raw,width=640,height=480" ! xvimagesink
inter: Whitespace cleanup
inter: Clean up surfaces after the last user is gone
interaudio: Properly pass through caps from the sink to the source

Otherwise a magic capsfilter after the source is required with
exactly the same caps as the input.
interaudio: Use GST_DEBUG_OBJECT() instead of GST_DEBUG()
interaudio: Fix timestamp, latency and period handling
inter: Truncate the caps as part of the fixating
intervideo: Add support for arbitrary video formats
interaudio: Add support for arbitrary raw audio formats
dash: Remove obsolete README.

https://bugzilla.gnome.org/show_bug.cgi?id=738848
gl: propogate other-context using GstContext
glmixer: override the accept caps query in order to 'convert' capsfeatures

Otherwise, it is only possible for the sink pads and the src pads to
have the exact same caps features.  We can convert from any feature
to another feature so support that.