meson: add options to disable external deps
Doesn't provide options for everything or for groups of things, so doesn't map 1:1 to configure, but allows us to easily build an ffmpeg without external library deps by passing --auto-features=disabled.
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Doesn't provide options for everything or for groups of things, so doesn't map 1:1 to configure, but allows us to easily build an ffmpeg without external library deps by passing --auto-features=disabled.