Header / Trailer Comment Typos for M4 macros

Hi hackers,

While poking at the build system I stumbled upon some trivial trailer
comment inconsistencies in config/c-compiler.m4. They can be fixed
either way: either changing the macro names or changing the comment. PFA
a patch that keeps the macro names.

In hindsight though, it seems that PGAC_PRINTF_ARCHETYPE was meant to be
PGAC_C_PRINTF_ARCHETYPE, judging by a half-second squint of surrounding,
similar macros. Thoughts?

Also in hindsight: it seems that, as suggested in the trailer typo,
PGAC_PROG_CXX_VAR_OPT (a la the C version PGAC_PROG_CC_VAR_OPT) would be
a good addition if we ever want to add the negative warning flags (for
starter, Wno-unused-command-line-argument for clang++) to CXXFLAGS, but
I assume it wasn't there in the final patch because we didn't use it
(presumably because the patch was minimized?). Thoughts?

Cheers,
Jesse