Require C++ Installation after using PBG_INSTALL_CPP=1 pip install .

## Steps to reproduce

  1. use PBG_INSTALL_CPP=1 pip install .
  2. run fb15k.py


## Observed Results

  * The script notifies that RuntimeError: GPU support requires C++ installation: install with C++ support by running `PBG_INSTALL_CPP=1 pip install .` even though I use the install command before.


Could someone please help me with this? Thanks!
