#if and the defined stuff are in even the older c standards, but it's
not defined to be part of the compiler (in plan 9, it's still in a separate cpp,
and usually that's not used, except with pcc, but see the -p option in 2c(1))

early on, cpp was fairly simple minded, part of the cc command(!), and
a c file had to have # at the start to be macro-processed.

the rot really set in with reiser
	This new version [of the preprocessor] was written by John F. Reiser and is from 5 to 12
	times faster than the old.
(but the input language was considerably more elaborate, and didn't people use it!)