Conditionally check if dune-common python package is installed

Closes #313

cmake/scripts/run-in-dune-env.sh.in

@@ -8,11 +8,16 @@ else
   exit 77
 fi
 
-# test if build directory matches installed dune python packages
-python -m dune checkbuilddirs @PROJECT_NAME@ @CMAKE_BINARY_DIR@
-if [ $? -eq 0 ] ; then
-  "$@"
-else
-  echo "Dune python package could not be found."
-  exit 77
+# check if dune-common python package is installed
+if python -m dune --help &> /dev/null; then
+  # test if build directory matches installed dune python packages
+  python -m dune checkbuilddirs @PROJECT_NAME@ @CMAKE_BINARY_DIR@
+  RESULT=$?
+  if [ $RESULT -ne 0 ] ; then
+    echo "Dune python package could not check build directories"
+    exit $RESULT
+  fi
 fi
+
+# execute the command
+"$@"