m4: add ACX_FC_MODULE_SEPARATE_SUBMOD.

configure

@@ -3541,6 +3541,53 @@ fi
          { $as_echo "$as_me:${as_lineno-$LINENO}: result: $acx_tmp$acx_fc_module_subname.$acx_cv_fc_module_subnaming_ext" >&5
 $as_echo "$acx_tmp$acx_fc_module_subname.$acx_cv_fc_module_subnaming_ext" >&6; }
 
+fi
+
+         { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether Fortran compiler generates submodule files for modules" >&5
+$as_echo_n "checking whether Fortran compiler generates submodule files for modules... " >&6; }
+if ${acx_cv_fc_module_separate_submod+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  acx_cv_fc_module_separate_submod=unknown
+      if test "x$acx_cv_fc_module_subnaming_ext" = x"$acx_cv_fc_module_naming_ext"; then :
+  acx_cv_fc_module_separate_submod=no
+else
+  as_dir=conftest.dir; as_fn_mkdir_p
+         cd conftest.dir
+         cat > conftest.$ac_ext <<_ACEOF
+      module conftest_module
+      implicit none
+      public
+      interface
+      module subroutine conftest_routine
+      end subroutine
+      end interface
+      end module
+_ACEOF
+if ac_fn_fc_try_compile "$LINENO"; then :
+  if test "x$acx_cv_fc_module_naming_upper" = xyes; then :
+  acx_fc_module_name='CONFTEST_MODULE'
+else
+  acx_fc_module_name='conftest_module'
+fi
+            if test -f "$acx_fc_module_name.$acx_cv_fc_module_subnaming_ext"; then :
+  acx_cv_fc_module_separate_submod=yes
+else
+  acx_cv_fc_module_separate_submod=no
+fi
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+         cd ..
+         rm -rf conftest.dir
+fi
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $acx_cv_fc_module_separate_submod" >&5
+$as_echo "$acx_cv_fc_module_separate_submod" >&6; }
+   if test "x$acx_cv_fc_module_separate_submod" = xunknown; then :
+  { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+$as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error $? "unable to detect whether Fortran compiler generates submodule files for modules
+See \`config.log' for more details" "$LINENO" 5; }
 fi
 
       { $as_echo "$as_me:${as_lineno-$LINENO}: checking for Fortran compiler flag needed to specify search paths for the \"INCLUDE\" statement" >&5

configure.ac

@@ -68,6 +68,8 @@ ACX_FC_MODULE_NAMING(
 
 ACX_FC_MODULE_SUBNAMING
 
+ACX_FC_MODULE_SEPARATE_SUBMOD
+
 ACX_FC_INCLUDE_FLAG(
   [AC_SUBST([FC_INC_FLAG], [$acx_cv_fc_ftn_include_flag])])
 ACX_FC_INCLUDE_ORDER(

m4/acx_fc_module.m4

@@ -276,6 +276,55 @@ submodule file naming template])])],
            [$acx_tmp$acx_fc_module_subname.$acx_cv_fc_module_subnaming_ext])
          $1])])
 
+# ACX_FC_MODULE_SEPARATE_SUBMOD([ACTION-IF-SUCCESS],
+#                               [ACTION-IF-FAILURE = FAILURE])
+# -----------------------------------------------------------------------------
+# Checks whether the Fortran compiler generates separate submodule files for
+# the root ancestor modules.
+#
+# If successful, runs ACTION-IF-SUCCESS, otherwise runs ACTION-IF-FAILURE
+# (defaults to failing with an error message).
+#
+# The result is cached in the acx_cv_fc_module_separate_submod variable. If the
+# compiler generates separate submodule files for the root ancestor modules,
+# acx_cv_fc_module_separate_submod is "yes", and "no" otherwise. If the
+# detection fails, acx_cv_fc_module_separate_submod is set to "unknown".
+#
+AC_DEFUN([ACX_FC_MODULE_SEPARATE_SUBMOD],
+  [AC_LANG_ASSERT([Fortran])dnl
+   AC_REQUIRE([ACX_FC_MODULE_NAMING])dnl
+   AC_REQUIRE([ACX_FC_MODULE_SUBNAMING])dnl
+   AC_CACHE_CHECK(
+     [whether Fortran compiler generates submodule files for modules],
+     [acx_cv_fc_module_separate_submod],
+     [acx_cv_fc_module_separate_submod=unknown
+      AS_VAR_IF([acx_cv_fc_module_subnaming_ext],
+        ["$acx_cv_fc_module_naming_ext"],
+        [acx_cv_fc_module_separate_submod=no],
+        [AS_MKDIR_P([conftest.dir])
+         cd conftest.dir
+         AC_COMPILE_IFELSE([AC_LANG_SOURCE(
+[[      module conftest_module
+      implicit none
+      public
+      interface
+      module subroutine conftest_routine
+      end subroutine
+      end interface
+      end module]])],
+           [AS_VAR_IF([acx_cv_fc_module_naming_upper], [yes],
+              [acx_fc_module_name='CONFTEST_MODULE'],
+              [acx_fc_module_name='conftest_module'])
+            AS_IF(
+              [test -f "$acx_fc_module_name.$acx_cv_fc_module_subnaming_ext"],
+              [acx_cv_fc_module_separate_submod=yes],
+              [acx_cv_fc_module_separate_submod=no])])
+         cd ..
+         rm -rf conftest.dir])])
+   AS_VAR_IF([acx_cv_fc_module_separate_submod], [unknown], [m4_default([$2],
+     [AC_MSG_FAILURE([unable to detect whether Fortran compiler generates dnl
+submodule files for modules])])], [$1])])
+
 # ACX_FC_MODULE_CHECK(MODULE-NAME,
 #                     [ACTION-IF-SUCCESS],
 #                     [ACTION-IF-FAILURE = FAILURE])