Makefile: allow optional local rules

I use Makefile.local to add my own rules for submitting a build
to Coverity. As those rules contain login credentials, they should
be kept private. Including Makefile.local (if present) seems to be
the best way to achieve that.

Also, add Makefile.local to .gitignore to make sure it never gets
commited into repo.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

.gitignore

@@ -15,6 +15,7 @@ criu
 cscope*
 tags
 TAGS
+Makefile.local
 syscall-x86-64.S
 include/syscall.h
 include/syscall-codes.h

Makefile

@@ -351,3 +351,6 @@ docker-test:
 	docker run --rm -it --privileged criu ./test/zdtm.sh -C -x tcp6 -x tcpbuf6 -x static/rtc -x cgroup -x mountpoint
 
 .DEFAULT_GOAL	:= all
+
+# include optional local rules
+-include Makefile.local