Add USE_INTERNAL_LIBS, a default for USE_INTERNAL_*

Linux distributions that want to link dependencies externally will
generally want to link (almost) every dependency externally; similarly,
minimal-dependency builds that want to use the embedded copies of
dependencies will generally want to do so for (almost) every dependency.
Make it easier to choose one of those by setting USE_INTERNAL_LIBS=0
or USE_INTERNAL_LIBS=1, respectively.

The default can still be overridden per-dependency; for instance,
"make USE_INTERNAL_LIBS=0 USE_INTERNAL_OPUS=1" will use the system
version of everything except Opus.

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