Add chromecast_build gyp variable and add rules when the variable is set to 1.

build/linux/system.gyp

Index: build/linux/system.gyp
diff --git a/build/linux/system.gyp b/build/linux/system.gyp
index dddf86a4eb50270ca93b9ff43f73ce18f16eb8ce..a60b3d55eeb67e3f37abcc2d7c6fcfb7edb47abd 100644
--- a/build/linux/system.gyp
+++ b/build/linux/system.gyp
@@ -1049,5 +1049,26 @@
         }],
       ],
     },
+    {
+      'target_name': 'directfb',
+      'type': 'none',
+      'conditions': [
+        ['_toolset=="target"', {
+          'direct_dependent_settings': {
+            'cflags': [
+              '<!@(<(pkg-config) --cflags directfb)',
+            ],
+          },
+          'link_settings': {
+            'ldflags': [
+              '<!@(<(pkg-config) --libs-only-L --libs-only-other directfb)',
+            ],
+            'libraries': [
+              '<!@(<(pkg-config) --libs-only-l directfb)',
+            ],
+          },
+        }],
+      ],
+    },
   ],
 }