Properly detect python header files

[yoonghm]

The original code use python -c "import sys; print(sys.prefix)" which did not show actual directory for include file.

The proper way is python -c "from sysconfig import get_paths as gp; print(gp()['include'])"

[ali5h]

with this patch i see the change in project-config.jam but g++ calls still use the wrong include path for python include.

[mclow]

A nit: We don't take PRs against the master branch. We take them against develop, then let the tests run, and merge to master.

[jsharpe]

This patch is incorrect - the real issue looks to be in boost build - if you don't fully specify version, includes and libraries in the python script it ignores the given version and tries to infer it from sys.version which is missing the m part of the version string.

[jsharpe]

See boostorg/build#481

[likan999]

I don't see how this could solve the problem. The issue is python.jam that sets the path to $(prefix)/include/python$(version).

[Flamefire]

The code looks correct as-is: $PYTHON_ROOT gets set to the root of the Python install, so this PR would set it to a wrong value