Build Cython modules with default compiler (not DPC++)

[PokhodenkoSA]

I think next step (in another PR) should be to try allow Python to build Cython modules with default compiler (not DPC++).
DPC++ is needed only in backend.

Originally posted by @PokhodenkoSA in #168 (comment)

[diptorupd]

@PokhodenkoSA I fixed this a while back in our setup.py. Can you confirm?

[PokhodenkoSA]

@diptorupd could you please attach PR with which you fixed this issue?

[diptorupd]

@PokhodenkoSA We originally were setting the CC and CXX to dpcpp in the build.sh. #168 removed the CC and CXX from build.sh, but added these two lines to the setup.py

    os.environ["CC"] = DPCPP_ROOT + "/bin/clang"
    os.environ["CXX"] = DPCPP_ROOT + "/bin/clang++"

#270 removed these two lines so that we do not use dpcpp to build the cython extensions.

[PokhodenkoSA]

Then I think it could be closed.