configure: tidy up comments

[vszakats]

• convert # comments to dnl, except copyright headers, and inline
  comments in curl-complilers.m4.
• drop empty comments.
• drop line-ending dnl markers. (except zz40-xc-ovr.m4 where it's
  used to produce a comment in configure.)
• replace dnl line with C comment in AC_CHECK_HEADERS().

Verified to produce the same configure script except empty lines,
# comments, and C comments, with autoreconf 2.72.

Cherry-picked from #21000

---

• drop dnl → # changes.

[dfandrich]

What's the reason for this? dnl is the accepted way to do comments in m4 and reduces the size of the output files.

[vszakats]

The idea is to use the same comment-style in all files. The patch set first switches
everything to dnl, then switches all of them to #.

The configure size advantage with dnl is 6% (88KB). While it also makes
it more readable due to the retained comments. # is less weird and more
familiar (IMO) than dnl, and supports inline comments.

I prefer # somewhat more. We can go either way.

[bagder]

personally, I don't think unifying the comments is worth the work. I don't think they cause trouble to anyone

[vszakats]

The work is already done, but since I sense we'd prefer staying with dnl,
I'll rewind to the point where # comments are swapped to dnl (except
for the rare inline ones), drop a bunch of empty comments and
accidental-looking line-ending dnls, and a keep a couple of tidy-ups
for robustness.