> "Also, does Unix really have more than one C compiler?"
GCC is pretty darn dominant, but as Daniel says, there are others.
With some regularity I use GCC, Intel's, and Clang. At one point or another I've also used Sun's (I'm not sure I have access to a Sun machine any more) as well as Pathscale's. Apparently I also have access to the Portland Group compilers... but I am not sure I've ever used them.
> "This makes me wonder what competing pre-C99 compilers output."
I tried GCC 2.95 (1999), but it just picks up the modern libc. (I wish that Sparc machine were up... I've got 2.7 that'll run on that and Alpha. :-))