Fix for zig build

[justsmth]

Issues

• Failure to cross-compile for Windows on macOS with cargo-zigbuild aws-lc-rs#512

Description of changes:

Zig compiler looks like both MINGW and CLANG. When building with zig, it fails due to the definition being set here:

...
  <command line>:9:9: error: '_WIN32_WINNT' macro redefined [-Werror,-Wmacro-redefined]
      9 | #define _WIN32_WINNT _WIN32_WINNT_WIN7
        |         ^
  <command line>:2:9: note: previous definition is here
      2 | #define _WIN32_WINNT 0x0a00
        |         ^
  In file included from <built-in>:416:
  <command line>:9:9: error: '_WIN32_WINNT' macro redefined [-Werror,-Wmacro-redefined]
      9 | #define _WIN32_WINNT _WIN32_WINNT_WIN7
        |         ^
  <command line>:2:9: note: previous definition is here
      2 | #define _WIN32_WINNT 0x0a00
        |         ^
  1 error generated.

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 78.80%. Comparing base (b7917d9) to head (8569cd3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2668      +/-   ##
==========================================
- Coverage   78.81%   78.80%   -0.02%     
==========================================
  Files         667      667              
  Lines      113982   113983       +1     
  Branches    16028    16030       +2     
==========================================
- Hits        89840    89823      -17     
- Misses      23367    23386      +19     
+ Partials      775      774       -1     

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