Here's a new patch that I believe should fix this issue.

It modifies Makefile.pre.in to include a few additional dependency
declarations for source files that depend on Include/graminit.h and
Include/Python-ast.h, as well as moving encoding_decl to the top of
Grammar/Grammar.

This should ensure that changes to the syntax and/or AST nodes will not
cause cryptic errors.

Longer term, it would be great to remove the parser's dependency on
graminit.h to ensure this sort of problem is never an issue.