[R] Allow JSON to be an optional component

[asfimport]

JSON support requires RapidJSON, a third-party dependency that might not always be available. Particularly for offline static builds (ARROW-12981), it would be nice to allow ARROW_JSON=OFF.

Here's the relevant section of ThirdpartyToolchain.cmake:

if(ARROW_JSON)
  set(ARROW_WITH_RAPIDJSON ON)
endif()

And the relevant section of the build_arrow_static.sh script.

As Neal mentioned, there's more to do than just replacing -DARROW_JSON=ON with -DARROW_JSON=$\{ARROW_JSON:-ON}. "We'll have to conditionally build some of the bindings like we do with dataset and parquet, and we'll have to conditionally skip tests."

Reporter: Karl Dunkle Werner / @karldw
Assignee: Karl Dunkle Werner / @karldw

PRs and other links:

• GitHub Pull Request #11046

_{Note: This issue was originally created as ARROW-13768. Please see the migration documentation for further details.}

[asfimport]

Ian Cook / @ianmcook:
Issue resolved by pull request 11046
#11046