Because mandoc_aux.h and mandoc.h use __attribute__, all files that

Because mandoc_aux.h and mandoc.h use __attribute__, all files that
include mandoc_aux.h or mandoc.h need to include config.h, too.
It is suspected that for example IRIX needs this, or it is likely
to throw errors in these files because the system compiler doesn't
understand __attribute__.
Issue reported by Kazuo Kuroi <kazuo at irixnet dot org>.