Improve handling of CFLAGS #1501

Job	Run time
apidiff (aws-lc-sys, branch)	2m 58s
apidiff (aws-lc-sys, published)	3m 15s
apidiff (aws-lc-fips-sys, branch)	3m 26s
bindgen-dependency (ubuntu-latest)	17s
apidiff (aws-lc-fips-sys, published)	3m 56s
bindgen-dependency (macos-13)	42s
apidiff (aws-lc-rs, branch)	2m 53s
bindgen-dependency (macos-14-xlarge)	38s
apidiff (aws-lc-rs, published)	4m 37s
bindgen-fips-dependency (ubuntu-latest)	16s
bindgen-fips-dependency (macos-13)	39s
bindgen-dependency (windows-latest)	52s
clippy (aws-lc-rs, --features bindgen,unstable)	1m 12s
bindgen-fips-dependency (macos-14-xlarge)	34s
dependency-review	15s
clippy (aws-lc-rs, --features bindgen,unstable,fips)	1m 47s
clippy (aws-lc-rs, --no-default-features --features aws-lc-sys)	47s
Resolve the dependencies to the minimum SemVer version (aws-lc-rs)	41s
rustfmt	25s
bindgen-fips-dependency (windows-latest)	54s
Resolve the dependencies to the minimum SemVer version (aws-lc-sys)	43s
udeps	7m 41s
Resolve the dependencies to the minimum SemVer version (aws-lc-fips-sys)	58s
metadata-checks	34s
semver-checks	1m 16s
	42m 16s

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