make check fails if no en_US.iso88591 locale

[Neil Jerram]

make check fails for me in regexp.test:

  ...
  Running regexp.test
  guile: uncaught throw to unresolved: ()

because I don't have an en_US.iso88591 locale installed, and so

  (with-locale "en_US.iso88591" ...)

throws an 'unresolved exception.

I can allow make check to complete by changing that line to

  (false-if-exception (with-locale "en_US.iso88591"

but I doubt that's the best fix.  Is the "en_US.iso88591" locale
actually important for the enclosed tests?

Thanks,
        Neil