Fix GCC preprocessor environment variable #726

Job	Run time
build	22s
check-doc	35s
check	9s
setup_concurrency	1s
cross-build (wpilib/roborio-cross-ubuntu:2024-22.04-py312)	6m 12s
cross-build (wpilib/raspbian-cross-ubuntu:bullseye-22.04-py312)	7m 59s
test (windows-latest, 3.8, x86)	6m 55s
cross-build (wpilib/aarch64-cross-ubuntu:bullseye-22.04-py312)	8m 8s
test (windows-latest, 3.8, x64)	6m 26s
test (windows-latest, 3.9, x86)	7m 28s
test (windows-latest, 3.9, x64)	8m 36s
test (windows-latest, 3.10, x86)	9m 13s
test (windows-latest, 3.10, x64)	7m 29s
test (windows-latest, 3.11, x86)	7m 35s
test (windows-latest, 3.11, x64)	6m 54s
test (windows-latest, 3.12, x86)	8m 45s
test (windows-latest, 3.12, x64)	7m 10s
test (macos-latest, 3.8, x64)	5m 54s
test (macos-latest, 3.9, x64)	4m 5s
test (macos-latest, 3.10, x64)	4m 17s
test (macos-latest, 3.11, x64)	12m 53s
test (macos-latest, 3.12, x64)	5m 44s
test (ubuntu-20.04, 3.8, x64)	5m 25s
test (ubuntu-20.04, 3.9, x64)	5m 37s
test (ubuntu-20.04, 3.10, x64)	4m 34s
test (ubuntu-20.04, 3.11, x64)	5m 2s
test (ubuntu-20.04, 3.12, x64)	5m 47s
publish	0s
	2h 39m 15s

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