fix: support building with MinGW on Windows 7

[justsmth]

Issues:

Addresses:

• Windows 7 support for MinGW #1111

Description of changes:

Enable x86_64-win7-windows-gnu (and similar win7 MinGW triples) to build and run on Windows 7 by routing CRYPTO_sysrand through BCryptGenRandom instead of ProcessPrng.

The upstream C source (crypto/rand_extra/windows.c) guards the Win7-compatible code path with !defined(__MINGW32__), so MinGW builds always take the ProcessPrng path — which aborts on Windows 7 because that API only exists on Windows 8+. This change works around the guard by defining AWSLC_WINDOWS_7_COMPAT directly from the build script for win7-gnu targets, and explicitly links bcrypt (since MinGW ignores the MSVC #pragma comment(lib, ...) in the source).

Call-outs:

This is a temporary workaround. The proper fix is aws/aws-lc#3239, which drops the !defined(__MINGW32__) guard upstream. Once that merges and we bump the submodule, the AWSLC_WINDOWS_7_COMPAT define can be removed. The _WIN32_WINNT and bcrypt link will still be needed for the CC builder path.

The existing win7-msvc handling was refactored into the same if target().contains("-win7-windows-") block so both toolchains share the _WIN32_WINNT=0x0601 define in one place.

Testing:

Build-tested with cargo check -p aws-lc-sys. Full validation requires a MinGW cross-compile targeting x86_64-win7-windows-gnu, which will be exercised once the upstream fix and a CI job are in place.

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 92.71%. Comparing base (c358484) to head (eef2aad).
⚠️ Report is 378 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1120      +/-   ##
==========================================
- Coverage   95.80%   92.71%   -3.09%     
==========================================
  Files          61       71      +10     
  Lines        8143    10248    +2105     
  Branches        0    10248   +10248     
==========================================
+ Hits         7801     9501    +1700     
- Misses        342      451     +109     
- Partials        0      296     +296     

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