[DRAFT] Use CFLAGS for include dirs

[justsmth]

Issues

See: https://github.com/rust-lang/cc-rs/pull/1043

Description of changes:

• cc utilizes the CFLAGS environment variable to obtain compiler arguments. The include directories specified in CFLAGS come before and take precedence over the include directories specified by our project.

Call-outs:

• If this issue is corrected upstream in cc-rs, the CFLAGS initialization in this PR is not needed, but it should not cause problems with the build.

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.

[justsmth]

My upstream PR to address this was merged. I would expect this fix to appear in the next (1.0.96?) release of cc.

I'll leave this PR in draft until the upstream fix is released.