portability: gcc sanitizer detection + POSIX strcasecmp + empty-TU se… #137

Job	Run time
macos-14	8m 27s
ubuntu-24.04	8m 33s
windows-2022	8m 46s
ubuntu-24.04-arm	8m 34s
external-test-suite (clojure-test-suite)	0s
perf-gate (linux)	0s
cross-compile (stencil-fresh on every target)	0s
	34m 20s

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