Improve handling of CFLAGS

[justsmth]

Issues

Addresses:

• Could not compile on Windows with crate rustls with config debug="limited" #707
• -sys: issues with CFLAGS from environment being passed through to CMake #710
• CMakeLists.txt wrong minimum required version #716

Description of changes:

• Previously we "harvested" the compiler options used by cc to populate CFLAGS. This is unnecessary, as it's also done by cmake-rs. This change identifies any supplementary compiler options as BuildOption values. These can be applied to either a cc or cmake build.
• cmake-rs strips optimization and debug options out of the CFLAGS environment variable. This can cause build failure when the build environment produces warnings due to lack of these optimizations.
• Aligning with the change made in AWS-LC, we're also bumping our CMake minimum version for v3.5.
• Fix for macro definitions applied after compiler checks.
• Defer the setting of CMAKE_BUILD_TYPE to cmake-rs

Callouts

• A test replicating the scenario in #710 is being prepared in #721.
• rustls integ test failures are being address in #717.

Testing

• Added test reproducing the scenario from #707
• Added test for various versions of CMake. #716

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 92.86%. Comparing base (c358484) to head (741f80b).
Report is 181 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #712      +/-   ##
==========================================
- Coverage   95.80%   92.86%   -2.94%     
==========================================
  Files          61       70       +9     
  Lines        8143     9561    +1418     
  Branches        0     9561    +9561     
==========================================
+ Hits         7801     8879    +1078     
- Misses        342      413      +71     
- Partials        0      269     +269     

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