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).

PC/pyconfig.h

@@ -694,6 +694,6 @@ Py_NO_ENABLE_SHARED to find out.  Also support MS_NO_COREDLL for b/w compat */
 #define HAVE_INET_PTON 1
 
 /* framework name */
-#define PYTHONFRAMEWORK ""
+#define _PYTHONFRAMEWORK ""
 
 #endif /* !Py_CONFIG_H */