Handle Windows not supporting static array dimension

[torben-hansen]

Issues:

CryptoAlg-2241

Description of changes:

awslabs/s2n-bignum#98 handles Windows not supporting static array dimensions. We have our own header files with function declarations for s2n-bignum. Hence that work was never ported automatically.

Adopt same detection approach as in referenced PR. MSVC doesn't support it, neither does C++. So, try to detect those cases as well as if variable-length arrays aren't supported.

Previous solution

Support it now but use our own macro OEPNSSL_WINDOWS to determine whether we are executing a Windows build. Take a dependency on target.h because this is minimal. Also teach the s2n-bignum file pro-processing where to find target.h even though I don't think this is strictly necessary; non of the pre-processed files include the header file and everywhere the header file is included will have at least base.h. But do it anyway.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.51%. Comparing base (49215e8) to head (d6d2397).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1912   +/-   ##
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  Coverage   78.50%   78.51%           
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  Files         585      585           
  Lines       99681    99681           
  Branches    14265    14271    +6     
=======================================
+ Hits        78257    78260    +3     
+ Misses      20788    20785    -3     
  Partials      636      636           

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