fix race with fresh perforce checkouts

R=r
DELTA=15  (9 added, 0 deleted, 6 changed)
OCL=20977
CL=20981

src/cmd/gc/Makefile

@@ -39,13 +39,22 @@ y.tab.h: $(YFILES)
 y.tab.c: y.tab.h
 	test -f y.tab.c && touch y.tab.c
 
+# the test here checks whether we have 6g at all.
+# if so, use it.  if not, just use the sysimport.c we have.
+# this happens on fresh perforce checkouts where
+# sysimport.c ends up with an older time stamp
+# than sys.go (or unsafe.go or mksys.c).
 sysimport.c:	sys.go unsafe.go mksys.c
-	gcc -o mksys mksys.c
-	6g sys.go
-	6g unsafe.go
-	./mksys sys >_sysimport.c &&\
-		./mksys unsafe >>_sysimport.c &&\
-		mv _sysimport.c sysimport.c
+	if test -x $(BIN)/6g; then \
+		gcc -o mksys mksys.c; \
+		6g sys.go; \
+		6g unsafe.go; \
+		./mksys sys >_sysimport.c && \
+		./mksys unsafe >>_sysimport.c && \
+		mv _sysimport.c sysimport.c; \
+	elif test -f sysimport.c; then \
+		touch sysimport.c; \
+	fi
 
 clean:
 	rm -f $(OFILES) *.6 enam.c 6.out a.out y.tab.h y.tab.c $(LIB) _sysimport.c