A deeper issue here is that Parser/parsetok.c has a dependency on
graminit.h. The problem is that Parser/parsetok.c is a part of the
Parser/pgen program which is actually being used to *generate*
graminit.h in the first place.

This breaks Python whenever syntax is added to or removed from
Grammar/Grammar in such a way that the value of encoding_decl changes,
because the value of encoding_decl used by pgen is different to the
value used to build python itself.

A simple work around for those wishing to change the syntax is a "make;
make clean; make". It'd obviously be nice if the build were fixed, though.