Update to EMSDK 3.1.31

[wydengyre]

Surprising error on the build here. I'll return to investigate if nobody else has an immediate intuition as to why this would be the case:

src/lib.cpp:224:26: error: no member named 'format' in namespace 'std'
[775](https://github.com/robertknight/tesseract-wasm/actions/runs/4031562992/jobs/6930925265#step:5:776)
    auto hocr_doc = std::format(R"(<?xml version="1.0" encoding="UTF-8"?>

[robertknight]

std::format requires C++20. This was added to the build flags here: https://github.com/robertknight/tesseract-wasm/blob/33f20fc01c17afbd6abb4c569d1d04a32e2c6f28/Makefile#L168. The latest EMSDK version might require a change in how this flag is passed.

[wydengyre]

@robertknight fixed

[robertknight]

Huh, std::format is a newer addition than I thought in clang. I wonder why it compiled successfully with the previous version of EMSDK.

[wydengyre]

It looks possible that the previous version of Clang did still support the limited use of format here, and didn't hide that support behind the experimental flag. Don't have the time atm to track down those details though.

[robertknight]

Thanks for the update. I might reconsider the use of std::format as it is only used in one place. According to https://releases.llvm.org/15.0.0/projects/libcxx/docs/ReleaseNotes.html#new-features no major changes are expected though, so this will probably become unnecessary in a release or two.