[Python binding] --use-system-libxgboost isn't respected by the build

[yurivict]

--use-system-libxgboost in the install args:

python-3.7 setup.py install  --record /disk-samsung/freebsd-ports/misc/py-xgboost/work-py37/.PLIST.pymodtmp  --use-system-libxgboost --single-version-externally-managed --root=/disk-samsung/freebsd-ports/misc/py-xgboost/work-py37/stage

isn't respected and the line in python-package/setup.py

if USER_OPTIONS['use-system-libxgboost'][2]:

is evaluated to false.

FreeBSD 12,2

[trivialfis]

This might be a bug in setup tools.

[hcho3]

Closing, since XGBoost no longer users setuptools and has transitioned to pyproject.toml based installation.