improve DunePythonInstallPackage to handle missing pip

cmake/modules/DunePythonInstallPackage.cmake

@@ -40,11 +40,6 @@ function(dune_python_install_package)
     message(WARNING "Unparsed arguments in dune_python_install_package: This often indicates typos!")
   endif()
 
-  # Check for the presence of the pip package
-  if(NOT DUNE_PYTHON_pip_FOUND)
-    message(FATAL_ERROR "dune_python_install_package: Requested installations, but pip was not found!")
-  endif()
-
   set(PYINST_FULLPATH ${CMAKE_CURRENT_SOURCE_DIR}/${PYINST_PATH})
   if(EXISTS ${PYINST_FULLPATH}/setup.py.in)
     configure_file(${PYINST_PATH}/setup.py.in ${PYINST_PATH}/setup.py)
@@ -88,6 +83,16 @@ function(dune_python_install_package)
                       "${PYINST_FULLPATH}")
 
 
+  # Leave this function if no installation rules are required
+  if("${DUNE_PYTHON_INSTALL_LOCATION}" STREQUAL "none" AND NOT DUNE_PYTHON_VIRTUALENV_SETUP)
+    return()
+  endif()
+
+  # Check for the presence of the pip package
+  if(NOT DUNE_PYTHON_pip_FOUND)
+    message(FATAL_ERROR "dune_python_install_package: Requested installations, but pip was not found!")
+  endif()
+
   #
   # If requested, install into the configure-time Dune virtualenv
   #