Sync up some inconsistent comments in config/c-compiler.m4.

Sync up some inconsistent comments in config/c-compiler.m4.

Make header/trailer comments agree with the actual names of some macros.
These seem like legit names in earlier iterations of respective patches
(commit b779168ff "Detect PG_PRINTF_ATTRIBUTE automatically." and
commit 6869b4f25 "Add C++ support to configure.") but the macro had
been renamed out of sync with the header / trailer comment in the final
committed patch.

Even more nitpickily, make the dashed underlines agree with the lengths
of the macro names everyplace.  There doesn't seem to have been any
meeting of the minds previously on whether those should match or not,
but at least some people have been trying to make 'em match.

Jesse Zhang, Tom Lane

Discussion: https://postgr.es/m/CAGf+fX7DDyq6WfCy6X_KtD28MkbNBE6NkRi26fSf25dfUwX0zw@mail.gmail.com