- fix `nghttp2` build to also build the `nghttpx` application.
Restore required `libc-ares-dev`. Also confirm that `libev-dev` is
required too. Document these requirements.
Follow-up to 0455d8772a#18509
- explicitly enable `nghttpx` for the `nghttp2` build to make it fail if
requirements aren't met:
```
configure: error: applications were requested (--enable-app) but dependencies are not met.
```
- explicitly install brotli, zstd, zlib for the dependency builds.
Of these, zstd and zlib are preinstalled. zlib is required for
`nghttpx`. zstd and brotli doesn't seem to be used, but keep them
there just in case and to match the test env.
Follow-up to 0455d8772a#18509
- enable brotli for `nghttpx`. It doesn't change the tests, and also
cost almost nothing, so I figure why not.
Closes#18522
53f90cb3b5