Add CPPFLAGS to Makefiles #529

Job	Run time
build (gcc, 3.11, 3.1)	1m 28s
build (gcc, 3.11, 3.1, test-flags-override)	1m 18s
build (gcc, 3.11, 3.1, test-debug)	1m 21s
build (gcc, 3.11, 3.1, linker-bfd)	1m 30s
build (gcc, 3.11, 3.1, linker-gold)	1m 29s
build (gcc, 3.10, 3.1)	1m 27s
build (gcc, 3.9, 3.1)	1m 37s
build (gcc, 3.8, 3.1)	1m 36s
build (gcc, 3.7, 3.1)	1m 55s
build (gcc, pypy3.7, 3.1)	1m 54s
build (gcc, 3.11, 3.0)	2m 13s
build (gcc, 3.11, 2.7)	1m 59s
build (gcc, 3.11, 2.6)	1m 43s
build (gcc, 3.11, 2.5)	1m 38s
build (clang, 3.11, 3.1)	1m 32s
build (clang, 3.11, 3.1, test-flags-override)	1m 22s
build (clang, 3.11, 3.1, test-debug)	1m 25s
build (clang, 3.10, 3.1)	1m 35s
build (clang, 3.9, 3.1)	1m 38s
build (clang, 3.8, 3.1)	1m 36s
build (clang, 3.7, 3.1)	1m 38s
build (clang, pypy3.7, 3.1)	2m 12s
build (clang, 3.11, 3.0)	1m 50s
build (clang, 3.11, 2.7)	1m 46s
build (clang, 3.11, 2.6)	2m 0s
build (clang, 3.11, 2.5)	1m 53s
build (gcc, 3.11, 3.1, sanitizers)	2m 2s
	45m 37s

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