GH-23221: [C++] webassembly / pyodide / emscripten build support

[joemarshall]

This PR adds a cmake build preset for emscripten / pyodide, and updates the setup for pyarrow so that it is possible to build a webassembly version of libarrow, and a version of pyarrow which works on pyodide.

Things that changed:

1. Cmake preset ninja-release-emscripten-python
2. Platform/toolchain override for cmake on emscripten, to set various build parameters
3. Some fixes to cmake files - e.g. pyarrow can build with static libarrow now, and third party dependencies pass the toolchain file through.
4. pyarrow knows about ARROW_ENABLE_THREADING and doesn't use threads if they are not enabled in libarrow.
5. pyarrow sets defaults for inclusion of submodules based on their inclusion in the arrow build. e.g. pyarrow.parquet is built only if ARROW_PARQUET is set. This makes it possible to build in situations where you don't have access to set the build environment variables (e.g. in cross compiling situations like pyodide). It also seems to me to make sense - I can't see why you'd want to build pyarrow without parquet if you've built arrow with it, and the inverse is obviously not possible.

• Closes: [C++] Add/document configuration to build with WebAssembly #23221

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

In the case of PARQUET issues on JIRA the title also supports:

PARQUET-${JIRA_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - How to contribute patches

[github-actions]

⚠️ GitHub issue #23221 has been automatically assigned in GitHub to PR creator.

[joemarshall]

Also fixes #34996 once this is in the pyodide package tree.

[joemarshall]

@kou I'm not sure who is the right person to review this - it's a smidgen of python / python packaging changes and cmake changes.

[kou]

Can we split this pull request to the C++ part and then the Python part for easy to review?

[joemarshall]

@kou - I've split out the cmake changes into #37821.

Python changes in #37822

[joemarshall]

Closing this as all changes are in the other PRs