Part way to allowing "from __future__ import generators" to communicate
that info to code dynamically compiled *by* code compiled with generators
enabled.  Doesn't yet work because there's still no way to tell the parser
that "yield" is OK (unlike nested_scopes, the parser has its fingers in
this too).
Replaced PyEval_GetNestedScopes by a more-general
PyEval_MergeCompilerFlags.  Perhaps I should not have?  I doubted it was
*intended* to be part of the public API, so just did.
diff --git a/Python/future.c b/Python/future.c
index 70be26b..f0960ae 100644
--- a/Python/future.c
+++ b/Python/future.c
@@ -32,7 +32,7 @@
 		if (strcmp(feature, FUTURE_NESTED_SCOPES) == 0) {
 			ff->ff_nested_scopes = 1;
 		} else if (strcmp(feature, FUTURE_GENERATORS) == 0) {
-			/* OK; this is processed by the parser */
+			ff->ff_generators= 1;
 		} else if (strcmp(feature, "braces") == 0) {
 			PyErr_SetString(PyExc_SyntaxError,
 					"not a chance");
@@ -233,6 +233,7 @@
 	ff->ff_found_docstring = 0;
 	ff->ff_last_lineno = -1;
 	ff->ff_nested_scopes = 0;
+	ff->ff_generators = 0;
 
 	if (future_parse(ff, n, filename) < 0) {
 		PyMem_Free((void *)ff);