Updates for building art

Add art and libnativehelper to the root Blueprints file, and
add a warning used by art to the clang unknown flags list.

Change-Id: If282413103fa20fa66422e4330fbabff61b66144
diff --git a/cc/clang.go b/cc/clang.go
index 6b25d4f..f3ced68 100644
--- a/cc/clang.go
+++ b/cc/clang.go
@@ -23,6 +23,7 @@
 	"-Wno-unused-but-set-variable",
 	"-Wno-unused-but-set-parameter",
 	"-Wno-unused-local-typedefs",
+	"-Wunused-but-set-parameter",
 
 	// arm + arm64 + mips + mips64
 	"-fgcse-after-reload",
diff --git a/root.bp b/root.bp
index 2dd14d4..247c96c 100644
--- a/root.bp
+++ b/root.bp
@@ -1,7 +1,9 @@
 subdirs = [
+    "art",
     "build/blueprint",
     "build/soong",
     "bionic/*",
     "external/*",
+    "libnativehelper",
     "system/core/*",
 ]