bpo-29585: Fix sysconfig.get_config_var("PYTHONFRAMEWORK") (GH-2483)

`PYTHONFRAMEWORK` is defined in `Makefile` and it shoulnd't be used
in `pyconfig.h`.

`sysconfig.py --generate-posix-vars` reads config vars from Makefile
and `pyconfig.h`.  Conflicting variables should be avoided.

Especially, string config variables in Makefile are unquoted, but
in `pyconfig.h` are keep quoted.  So it should be private (starts with
underscore).