Unbreak "dist" target for fresh trees

Unbreak "dist" target for fresh trees
The previous hack to include Parser.hs in source distros broke the possibility
of doing a "make dist" in a fresh tree, i.e. one which has just been checked
out and configured, but *not* built. Of course you will need Happy for such a
source distro later, but at least the freedom to do this is important.

The ultimate goal should be that something like "make dist" will work in a
freshly checked out tree, with no prerequisite steps (this is very common in
most projects). We should move towards that goal, not away from it...

MERGE TO STABLE