Reportedly, some compilers warn about tests like "c < 0" if c is unsigned,
and hence complain about the character range checks I added in commit
3bb3f42f.  This is a bit of a pain since
the regex library doesn't really want to assume that chr is unsigned.
However, since any such reconfiguration would involve manual edits of
regcustom.h anyway, we can put it on the shoulders of whoever wants to
do that to adjust this new range-checking macro correctly.

Per gripes from Coverity and Andres.