Env var priority?

[NobodyXu]

zulip link

Hello! We have some users reporting build failures. The failure relates to some code we recently added that MUST be compiled w/o optimizations: https://github.com/aws/aws-lc/blob/main/third_party/jitterentropy/jitterentropy-library/src/jitterentropy-base.c#L46-L48

However, many build environments have something like CFLAGS=-O3 that is always present in the environment. Due to cc-rs prioritizing CFLAGS from the environment, aws-lc-sys is unable to override the -O3:

cc-rs/src/lib.rs

Line 2015 in 6fb2a8f

if let Some(flags) = &envflags {

This happens after #1403

[NobodyXu]

Proposed solution after discussion on zulip:

have a new function to turn off env vars like CFLAGS so that build scripts have full control of flags