safe_math.h: Check if GNUC is defined #2296

Job	Run time
basic_clang	5m 55s
basic_gcc	5m 52s
check_docs	56s
check_update	1m 18s
check-ansi	2m 34s
minimal	2m 49s
no-deprecated	5m 27s
non-caching	12m 44s
no-shared-macos (macos-13)	13m 56s
no-shared-ubuntu	4m 32s
no-shared-macos (macos-14)	3m 44s
self-hosted	0s
address_ub_sanitizer	11m 10s
external-tests-misc	7m 27s
enable_non-default_options	4m 41s
external-tests-providers	9m 8s
external-tests-pyca (1.51.0, 3.9)	6m 23s
full_featured	6m 2s
fuzz_tests	3m 2s
legacy	4m 47s
memory_sanitizer	18m 33s
no-legacy	5m 46s
out-of-readonly-source-and-install-macos (macos-13)	14m 48s
out-of-readonly-source-and-install-ubuntu	7m 30s
out-of-readonly-source-and-install-macos (macos-14)	6m 5s
threads_sanitizer	5m 0s
	2h 50m 9s

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