Revise documentation of AC_PROG_CC and comments on conformance checks.

Revise documentation of AC_PROG_CC and comments on conformance checks.

Makes the documentation of AC_PROG_CC consistent with the
documentation of AC_PROG_CXX.  Also removes a bunch of redundant text
from c.m4 and adds lists of the headers that *can* be used in the
conformance tests, so future hackers don’t have to look them up.

* doc/autoconf.texi (AC_PROG_CC): Make description consistent with
  description of AC_PROG_CXX.
* lib/autoconf/c.m4: Clean up some outdated or repetitive commentary
  and add lists of the freestanding headers above the code that needs
  to avoid using non-freestanding headers.