After GH-136 merged, PYTHONFRAMEWORK config variable are in Makefile and pyconfig.h.

When reading string variable from Makefile, quote is removed.
But when reading them from pyconfig.h, quote is not removed. And config variables in
pyconfig.h override config variables in Makefile.

So sysconfig.get_config_var("PYTHONFRAMEWORK") was changed '' to '""' in
non framework build, and 'PYTHON' to '"PYTHON"' in framework build.

I thought about unquoting string config variables in pyconfig.h, but there is one
string variables already and it may cause unintentional backward incompatibility.
(sysconfig.get_config_var('PY_FORMAT_SIZE_T') is '"z"').

So this pull request just rename PYTHONFRAMEWORK to _PYTHONFRAMEWORK
in pyconfig.h. Variables which name starts with _ is not exported to sysconfig.