[dunecontrol] Fixes error with installed old DUNE modules

If duncontrol detects an installed dune-common module, then it always
assumed that it contains a lib/dunecontrol
subdirectory. Unfortunately, this does not hold for older dune-common
modules and dunecontrol will error out later with a message like

/usr/local/lib/dunecontrol: file not found
Execution of dunecontrol terminated due to errors!

With this commit we fix this by actually checking whether the
lib/dunecontrol subdirectoy exists before adding it to
DUNE_CONTROL_PATH.

Thanks a lot to wuhao sife for reporting this problem.

lib/dunemodules.lib

@@ -211,7 +211,13 @@ setup_control_path() {
         fi
       done
     else
-       DUNE_CONTROL_PATH=$DUNE_CONTROL_PATH:$(pkg-config dune-common --variable=prefix)/lib/dunecontrol
+      NEW_DUNE_CONTROL_DIR=$(pkg-config dune-common --variable=prefix)/lib/dunecontrol
+      # There might be old version of DUNE lying around, that do not contain
+      # lib/dunecontrol, yet. To prevent failur at a later stage of dunecontrol
+      # we check that the directory really exists.
+      if test -d "$NEW_DUNE_CONTROL_DIR"; then
+        DUNE_CONTROL_PATH=$DUNE_CONTROL_PATH:$NEW_DUNE_CONTROL_DIR
+      fi
     fi
   fi
   # try to read DUNE_CONTROL_PATH from OPTS file