Add `--build` and `--emulator-x86` to `./mach bootstrap-android`
This allows not downloading dependencies that are not needed for a particular task.
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base-devel contains the last version of autoconf. However, some
dependencies of the project rely on autoconf 2.13. Without this change,
the build fails with the following message:
```
ERROR: Could not find autoconf 2.13
--- stderr
ERROR: Could not find autoconf 2.13
make: *** [makefile.cargo:143: maybe-configure] Error 1
thread 'main' panicked at 'assertion failed: result.success()', build.rs:110:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.
```
List additional keyboard shortcuts
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- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
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Add macOS gstreamer setup
This was obtained from the gstreamer website rather than looking for a minimal set of install targets.
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