disable builtin rules

We define all rules/targets ourselves, so the builtin ones just get in
our way and cause weirdness & slow things down.  Disable them.

Change-Id: I85075297322e3e0515141cc908a67065cbfe8e2c
Reviewed-on: https://weave-review.googlesource.com/3151
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
diff --git a/Makefile b/Makefile
index 965fd85..0eff8e1 100644
--- a/Makefile
+++ b/Makefile
@@ -2,6 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
+# Disable all builtin rules first as we don't use any of them (we define all
+# rules/targets ourselves, nor do we want to rely on them.
+MAKEFLAGS += --no-builtin-rules
 # Run make with BUILD_MODE=Release for release.
 BUILD_MODE ?= Debug