Warnings about rcs_id strings: let's settle this

[Zack Weinberg]

Joe Buck <jbuck@synopsys.com> writes:

> only for const variables?  NO.  Many old codes do not declare the rcsid
> const, and the fact that we have the flag that will warn about all unused
> static variables means that users can still find them all.

My understanding of the message raising that concern was that unused
non-const static variables were already covered by some other option.
I don't have a strong opinion here.  I do think that enabling this
warning at -Wextra is a good idea, as it is consistent with other
"unused thing which may be hard to get rid of" warnings.

zw