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).
diff --git a/configure b/configure
index ff8152d..3a28cad 100755
--- a/configure
+++ b/configure
@@ -3245,7 +3245,7 @@
 
 
 cat >>confdefs.h <<_ACEOF
-#define PYTHONFRAMEWORK "${PYTHONFRAMEWORK}"
+#define _PYTHONFRAMEWORK "${PYTHONFRAMEWORK}"
 _ACEOF