Move the source-selection macros to a common header #84

Job	Run time
ubuntu-latest/linux-g++	1m 40s
macos-13/clang	1m 52s
ubuntu-latest/gcc-no-math	1m 11s
ubuntu-latest/clang	1m 35s
ubuntu-latest/gcc-freestanding	1m 3s
	7m 21s

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