build: update Makefile to track source code dependencies better

Also update .hgignore to ignore y.output.

R=rsc
CC=golang-dev
https://golang.org/cl/5797061

.hgignore

@@ -33,6 +33,7 @@ misc/goplay/goplay
 misc/osx/*.pkg
 misc/osx/*.dmg
 src/cmd/6a/6a
+src/cmd/?a/y.output
 src/cmd/?l/enam.c
 src/cmd/cc/y.output
 src/cmd/dist/dist.dSYM

src/cmd/gc/Makefile

@@ -6,7 +6,7 @@ include ../../Make.dist
 
 install: y.tab.h builtin.c
 
-y.tab.h: go.y
+y.tab.h: go.y go.errors bisonerrors
 	bison -v -y -d go.y
 	# make yystate global, yytname mutable
 	cat y.tab.c | sed '/ int yystate;/d; s/int yychar;/int yychar, yystate;/; s/static const char \*const yytname/const char *yytname/; s/char const \*yymsgp/char *yymsgp/' >y1.tab.c