The emscripten compiler has alloca.h

[kroening]

Emscripten is a compiler that targets WASM. To enable cross-compilation with emscripten, allocainc.h now uses alloca.h for this compiler.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (52688d7) 79.09% compared to head (6a14010) 79.09%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8085   +/-   ##
========================================
  Coverage    79.09%   79.09%           
========================================
  Files         1701     1701           
  Lines       196625   196625           
========================================
  Hits        155527   155527           
  Misses       41098    41098           

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[peterschrammel]

Suggested change

	# include <alloca.h>
	# include <alloca.h>

[kroening]

clang-format won't like that.