(strtoimax): Guard declaration with
`#if !HAVE_DECL_STRTOIMAX', rather than just `#ifndef strtoimax'.
The latter fails because some systems (at least rs6000-ibm-aix4.3.3.0)
have their own, conflicting declaration of strtoimax in sys/inttypes.h.
(strtoumax): Likewise, for completeness (it wasn't necessary).