Use result of string manipulation test.

configure

@@ -29725,7 +29725,7 @@ $as_echo "yes" >&6; }
 else
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }
-
+enable_isoc=no
 fi
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
    ac_ext=c

configure.ac

@@ -245,7 +245,7 @@ AC_ARG_ENABLE([iso-c-interface],
                  [Create Fortran Interface via iso_c_bindings facility of F2003 [default=no].])],
               [enable_isoc=${enableval}],[enable_isoc=no])
 AS_IF([test x"$enable_isoc" = xyes],
-  [ACX_FC_CHECK_STRPTR_CONVERT],[enable_isoc=no])
+  [ACX_FC_CHECK_STRPTR_CONVERT(,[enable_isoc=no])],[enable_isoc=no])
 AM_CONDITIONAL([CREATE_ISOC],[test $enable_isoc = 'yes'])
 AC_SUBST([ENABLE_F2003_ISOC],[$enable_isoc])
 AM_SUBST_NOTMAKE([ENABLE_F2003_ISOC])