Restore present check for set_acc_host_or_device (!100)

The present check in the `set_acc_host_or_device` is heavily used in ICON and cannot be changed easily.
This problem comes from the discussion at https://gitlab.dkrz.de/icon/icon-mpim/-/merge_requests/563#note_276277


Revert the changes.

Approved-by: Marek Jacob <1129-b380572@users.noreply.gitlab.dkrz.de>
Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: bugfix

src/mo_fortran_tools.F90

@@ -2326,22 +2326,28 @@ CONTAINS
 
   SUBROUTINE assert_acc_host_only(routine_name, lacc)
     CHARACTER(len=*), INTENT(IN) :: routine_name
-    LOGICAL, INTENT(IN) :: lacc
+    LOGICAL, INTENT(IN), OPTIONAL :: lacc
 
 #ifdef _OPENACC
-    IF (lacc) THEN
-      CALL finish(routine_name, 'not supported on ACC device.')
+    IF (PRESENT(lacc)) THEN
+      IF (lacc) THEN
+        CALL finish(routine_name, ' not supported on ACC device.')
+      END IF
     END IF
 #endif
   END SUBROUTINE assert_acc_host_only
 
   SUBROUTINE assert_acc_device_only(routine_name, lacc)
     CHARACTER(len=*), INTENT(IN) :: routine_name
-    LOGICAL, INTENT(IN) :: lacc
+    LOGICAL, INTENT(IN), OPTIONAL :: lacc
 
 #ifdef _OPENACC
-    IF (.NOT. lacc) THEN
-      CALL finish(routine_name, 'not supported in ACC host mode.')
+    IF (.NOT. PRESENT(lacc)) THEN
+      CALL finish(routine_name, ' must not be called without lacc.')
+    ELSE
+      IF (.NOT. lacc) THEN
+        CALL finish(routine_name, ' not supported in ACC host mode.')
+      END IF
     END IF
 #endif
   END SUBROUTINE assert_acc_device_only
@@ -2361,11 +2367,13 @@ CONTAINS
 
   PURE SUBROUTINE set_acc_host_or_device(lzacc, lacc)
     LOGICAL, INTENT(OUT) :: lzacc
-    LOGICAL, INTENT(IN) :: lacc
+    LOGICAL, INTENT(IN), OPTIONAL :: lacc
 
     lzacc = .FALSE.
 #ifdef _OPENACC
-    lzacc = lacc
+    IF (PRESENT(lacc)) THEN
+      lzacc = lacc
+    END IF
 #endif
   END SUBROUTINE set_acc_host_or_device