Allow static and shared libs to be mixed (take 2)

[graebm]

This was previously reviewed here: #3467
That was merged to main, but then it broke many other tests, so the commit was reverted.
This PR is exactly the same thing, except LD_LIBRARY_PATH is no longer being tampered with.

I've run the s2nOmnibus tests on this branch, and they're all passing.

Resolved issues:

This is part of a fix for an issue from another repo: aws4embeddedlinux/meta-aws#191

Description of changes:

Currently, users cannot build a shared lib that uses a static libs2n.a, and they cannot build a static lib that uses a shared libs2n.so.

With this change, a library can use either static libs2n.a or shared libs2n.so. If both are static and shared libs2n are installed, BUILD_SHARED_LIBS serves as a tie-breaker.

Testing:

Added new unit test.

Did local testing:

• installed static libs2n.a and then built a shared library that used it.
• installed shared libs2n.so and then built a static library that used it.
• installed BOTH static libs2n.a and shared libs2n.so
  • Built my own shared library and confirmed that it used picked libs2n.so
  • Built my own static library and confirmed that it picked libs2n.a
• CodeBuild ad-hoc job

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

[dougch]

passed locally and in ad-hoc CodeBuild job.