Instead pass -Wl,exclude-libs,$lib for each library *NOT* mentioned
explicitly on the command line so those symbols aren't placed in
exported symbol tables.  Fixes numerous linking errors in dependant
libraries that are extremely likely to be unused directly.

This does however mean that any symbols and libraries that are used need
to be present in the command line explicitly using either
GSTREAMER_EXTRA_DEPS (for pkg-config) or -l$lib arguments.

Fixes gstreamer/cerbero#52
Fixes gstreamer/gst-libav#37