Improve handling of CFLAGS #1505

Job	Run time
apidiff (aws-lc-sys, branch)	2m 52s
apidiff (aws-lc-sys, published)	3m 12s
apidiff (aws-lc-fips-sys, branch)	3m 12s
apidiff (aws-lc-fips-sys, published)	4m 12s
apidiff (aws-lc-rs, branch)	2m 56s
clippy (aws-lc-rs, --features bindgen,unstable)	1m 15s
dependency-review	20s
apidiff (aws-lc-rs, published)	3m 8s
clippy (aws-lc-rs, --features bindgen,unstable,fips)	1m 35s
rustfmt	33s
bindgen-dependency (ubuntu-latest)	16s
bindgen-dependency (macos-13)	33s
Resolve the dependencies to the minimum SemVer version (aws-lc-rs)	47s
clippy (aws-lc-rs, --no-default-features --features aws-lc-sys)	52s
bindgen-fips-dependency (ubuntu-latest)	28s
Resolve the dependencies to the minimum SemVer version (aws-lc-sys)	46s
bindgen-dependency (macos-14-xlarge)	37s
bindgen-fips-dependency (macos-13)	45s
Resolve the dependencies to the minimum SemVer version (aws-lc-fips-sys)	1m 4s
bindgen-fips-dependency (macos-14-xlarge)	30s
bindgen-dependency (windows-latest)	52s
semver-checks	1m 22s
bindgen-fips-dependency (windows-latest)	48s
udeps	7m 48s
metadata-checks	28s
	41m 11s

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