Fix incremental rebuild after deleted scan.c.

Fix incremental rebuild after deleted scan.c.

It is important that the .stamp.c rule is checked first, before the
.l.c or .y.c rules (which do nothing) in order to trigger a rebuild
if scan.c or parse.c are inadvertently deleted.

Make considers suffix rules in the same order as the .SUFFIXES list.
Presently, this list is generated by Automake so we are at the mercy of
whatever order they happen to be in, which is perl sort order.  As far
as I can see it has always been this way, so the rebuild for scan.c has
probably never worked (but parse.c is OK, since .y sorts later).

Automake puts user-specified SUFFIXES first, so adding .stamp there
should suffice to resolve the problem.