Test for dangling rules in make checkbuild

Get a list of leaf nodes in the dependency graph
from ninja, and make sure none of them are in the
output directory.  This ensures that there are no
rules that depend on a file in the output directory
that doesn't have rule to generate it.  The check
will catch a common set of build failures where
a rule to generate a file is deleted (either by
deleting a module in an Android.mk file, or by
modifying the build system incorrectly).  These
failures are often not caught by a local incremental
build because the previously built files are still
present in the output directory.

Bug: 36843214
Bug: 68062417
Test: manual
Change-Id: I4933187e8b72f2ef0c32d18ffea756e2c6fa417c
diff --git a/ui/build/config.go b/ui/build/config.go
index 940bb2f..99ae2da 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -34,11 +34,12 @@
 	environ   *Environment
 
 	// From the arguments
-	parallel  int
-	keepGoing int
-	verbose   bool
-	dist      bool
-	skipMake  bool
+	parallel   int
+	keepGoing  int
+	verbose    bool
+	checkbuild bool
+	dist       bool
+	skipMake   bool
 
 	// From the product config
 	katiArgs     []string
@@ -206,6 +207,8 @@
 		} else {
 			if arg == "dist" {
 				c.dist = true
+			} else if arg == "checkbuild" {
+				c.checkbuild = true
 			}
 			c.arguments = append(c.arguments, arg)
 		}
@@ -313,6 +316,12 @@
 	panic("SetKatiSuffix has not been called")
 }
 
+// Checkbuild returns true if "checkbuild" was one of the build goals, which means that the
+// user is interested in additional checks at the expense of build time.
+func (c *configImpl) Checkbuild() bool {
+	return c.checkbuild
+}
+
 func (c *configImpl) Dist() bool {
 	return c.dist
 }