proposed patch for constant signed/unsigned comparison warning

[Jim Wilson]

	When rms originally installed this
	warning, it wasn't enabled with -Wall; it was enabled by the ``extra
	warning'' option -W.  This seems reasonable to me, though it's even
	more reasonable to separate it out into a separate -Wsign-compare
	option, as is done with my proposed patch.  The point, though, is that
	this warning shouldn't be enabled by -Wall.

It was Jason Merrill who added the -Wsign-compare option and made -Wall enable
it.  I asked him about this, and he said the purpose of this was only so that
the warnings could be turned off.  Previously, they were enabled by -W, and
there was no way to get the other -W warnings without also getting the
comparison warning.  -Wsign-compare was added so that the warnings could be
turned off.  Of course, now that there was a new variable, it needed to be
set someplace, and he choose to have -Wall set it, but there was no particular
reason for this choice.  Perhaps this was done because -Wall sets numerous
warn_* variables, but -W does not.

Since this particular change was not intentional, there does not seem to be
any reason why it must be retained.  I think we could make everyone happy
here if we changed this back so that -Wsign-compare is enabled by -W.
This seems to match the intent of -W, and also makes it easy for people
to still get the warnings.  And people can still disable the warning by
using -Wno-sign-compare.

I don't like the idea of only enabling the warning if -Wsign-compare is given.
The warning is very useful for some people, but they may never know they
need it if it is only enabled by some obscure option.  If -W enables it,
then people will still get the warnings if they use -W.

Or maybe we need a -Weverything option, which includes all warnings, even
those that we don't want enabled by -Wall?

Jim