Forward CMake variables to prebuilding dependencies

[sfod]

Issue #, if available:

When prebuilding dependencies, we need to pass certain CMake variables from parent cmake process to child cmake process (e.g. toolchain file or path to C compiler). CMake doesn't provide this functionality out of the box.

Description of changes:

This PR implements a new CMake function that checks CMake cache file and collects variables that may affect build process. aws_prebuild_dependency uses this new function, so no changes are required in the downstream projects.

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.63%. Comparing base (252e8c2) to head (8f8067c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1161   +/-   ##
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  Coverage   83.63%   83.63%           
=======================================
  Files          57       57           
  Lines        5953     5953           
=======================================
  Hits         4979     4979           
  Misses        974      974           

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[graebm]

Looking pretty good.
There might be a few more things we should look into (mentioned in comments), but when you're happy with it, I'm happy with it.