Add clang macro conditionals to eliminate compile noise on GCC. #1752

Job	Run time
Lint	57s
macOS 11 native [GOAL install] [GUI] [no depends]	8m 6s
ARM [GOAL install] [buster]	2m 59s
Win32	4m 34s
Win64	6m 47s
i386 Linux [GOAL install] [focal]	2m 46s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	8m 6s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	8m 6s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	8m 7s
	50m 28s

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