gst-libav: configure exits with no error status if configuring internal libav fails

Submitted by Alistair Buxton

Link to original bug (#794912)

Description

Here the configure for gst-libav runs the configure for libav:

https://cgit.freedesktop.org/gstreamer/gst-libav/tree/configure.ac#n464

If for some reason the libav configuration fails, the gst-libav configuration does not return an error status. This means if you are building from a script or Makefile, then that script will not realise that an error has occured and will continue to attempt to build gst-libav, with undefined results.

I encountered this in the context of 39df65e8. If I try to build from the commit prior to this one, libav configure will fail as it cannot find gas-preprocessor. Despite this, attempting to compile gst-libav still "succeeds" - in the sense that it also does not return an error. I have no idea what it actually produces, but my build scripts happily pack it up and report success.