* Also, test_plot cannot run in MacOS because of matplotlib & ipython's
  limit.
* Add missing dependency in setup.py.
* fix #2264

When using system python, "pip install" needs root previlege. Using virtualenv to avoid this.

In the original util.cmake. enable_virtualenv.c is always regenerated when cmake files are changed, which leads to the relinking of all the targets depends on paddle_utils.

Sometime FIND_PACKAGE(ProtoBuf) get an empty PROTOBUF_VERSION. In this case, ${PROTOBUF_VERSION} needs to be quoted.