multimedia/shaka-packager: Use system libraries

Build with USE_SYSTEM_DEPENDENCIES so the port links against the system
abseil, curl, mbedtls, libpng, libwebm, libxml2, nlohmann-json and
protobuf instead of the bundled copies.

Only the packager and mpd_generator tools are built; the unit test suite
(which requires a bundled mongoose and googletest) is skipped. A couple
of source patches adjust for the system abseil, whose MutexLock API is
older than the copy vendored by upstream.