Remove -sSUPPORT_LONGJMP=wasm and use of EMCC_CFLAGS when building xeus-cpp

[mcbarton]

Description

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Fixes # (issue)

Type of change

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• Bug fix
• New feature
• Requires documentation updates

Testing

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Checklist

• I have read the contribution guide recently

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.56%. Comparing base (be35eb6) to head (53e6101).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #803   +/-   ##
=======================================
  Coverage   79.56%   79.56%           
=======================================
  Files          11       11           
  Lines        4012     4012           
=======================================
  Hits         3192     3192           
  Misses        820      820           

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[anutosh491]

We expect emscripten to enable this -sSUPPORT_LONGJMP=wasm by default when we use wasm exceptions, so this should do !