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.

Change-Id: Ia29cd3d205e401dc3d04fbaf3cba2b01f2f4f0ca

Becasue of extern package.

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