ndk-build: don't advertise symbols from non-explicitly linked libraries
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 #52 (closed) Fixes gst-libav#37 (closed)