Fix bug in Fortran implicit int handling

* lib/autoconf/fortran.m4 (_AC_FC_IMPLICIT_NONE):
Fix typo in argument numbering.
Copyright-paperwork-exempt: yes

lib/autoconf/fortran.m4

diff --git a/lib/autoconf/fortran.m4 b/lib/autoconf/fortran.m4
index 4c39607..09459f0 100644
--- a/lib/autoconf/fortran.m4
+++ b/lib/autoconf/fortran.m4
@@ -1665,13 +1665,13 @@
 _AC_LANG_PREFIX[]FLAGS=$ac_fc_implicit_none_[]_AC_LANG_PREFIX[]FLAGS_save
 ])
 if test "x$ac_cv_[]_AC_LANG_ABBREV[]_implicit_none" = xunknown; then
-  m4_default([$3],
+  m4_default([$2],
     [AC_MSG_ERROR([no Fortran flag to disallow implicit declarations found], 77)])
 else
   if test "x$ac_cv_[]_AC_LANG_ABBREV[]_implicit_none" != xnone; then
     _AC_LANG_PREFIX[]FLAGS="$_AC_LANG_PREFIX[]FLAGS $ac_cv_[]_AC_LANG_ABBREV[]_implicit_none"
   fi
-  $2
+  $1
 fi
 ])# _AC_FC_IMPLICIT_NONE