Add clang macro conditionals to eliminate compile noise on GCC. #2821

Job	Run time
Lint	1m 0s
macOS 12 native [GOAL install] [GUI] [no depends]	21m 0s
ARM [GOAL install] [buster]	8m 9s
Win32	10m 3s
Win64	13m 54s
i386 Linux [GOAL install] [focal]	8m 8s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	16m 8s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	13m 56s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	9m 41s
	1h 41m 59s

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