add --force to pandas build error message

Author: max-sixty

pandas/__init__.py

@@ -27,8 +27,8 @@
     module = str(e).lstrip('cannot import name ')  # hack but overkill to use re
     raise ImportError("C extension: {0} not built. If you want to import "
                       "pandas from the source directory, you may need to run "
-                      "'python setup.py build_ext --inplace' to build the C "
-                      "extensions first.".format(module))
+                      "'python setup.py build_ext --inplace --force' to build "
+                      "the C extensions first.".format(module))
 
 from datetime import datetime
 from pandas.info import __doc__