I notice that AC_MINGW32 and friends don't cooperate with
AC_CANONICAL_HOST. A configure.in
AC_INIT
AC_CANONICAL_HOST
AC_MINGW32
AC_CYGWIN
AC_EMXOS2
processed with autoconf gives
configure.in:3: warning: AC_CANONICAL_HOST invoked multiple times
configure.in:4: warning: AC_CANONICAL_HOST invoked multiple times
configure.in:5: warning: AC_CANONICAL_HOST invoked multiple times
I see it was a deliberate change not to use AC_REQUIRE in those,
perhaps something else is wanted.
(I know these macros are obsolete, I only struck this running a recent
autoconf over a guile configure.in.)