Fixed the nasty bug where .depend file that exists somewhere in

the .PATH (but not in the ${.OBJDIR}) would result in a leak of
the ${OBJS}: ${SRCS:M*.h} dependency hint.

Spotted by:	fixing the broken gnu/usr.bin/cc/cc1obj build
MFC after:	1 day

share/mk/bsd.lib.mk

@@ -390,7 +390,7 @@ maninstall:
 
 .include <bsd.dep.mk>
 
-.if !exists(${DEPENDFILE})
+.if !exists(${.OBJDIR}/${DEPENDFILE})
 ${OBJS} ${STATICOBJS} ${POBJS} ${SOBJS}: ${SRCS:M*.h}
 .endif
 

share/mk/bsd.prog.mk

@@ -240,7 +240,7 @@ regress:
 
 .include <bsd.dep.mk>
 
-.if defined(PROG) && !exists(${DEPENDFILE})
+.if defined(PROG) && !exists(${.OBJDIR}/${DEPENDFILE})
 ${OBJS}: ${SRCS:M*.h}
 .endif