GN: Add default target

BUILD.gn

 # Copyright 2014 The LibYuv Project Authors. All rights reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS. All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("libyuv.gni")
 import("//testing/test.gni")
 
 config("libyuv_config") {
   include_dirs = [ "include" ]
   if (is_android && current_cpu=="arm64") {
     ldflags = [ "-Wl,--dynamic-linker,/system/bin/linker64" ]
   }
   if (is_android && current_cpu != "arm64") {
     ldflags = [ "-Wl,--dynamic-linker,/system/bin/linker" ]
   }
 }
 
+# This target is built when no specific target is specified on the command line.
+group("default") {
+  testonly = true
+  deps = [
+    ":libyuv",
+  ]
+  if (libyuv_include_tests) {
+    deps += [
+      ":compare",
+      ":convert",
+      ":cpuid",
+      ":libyuv_unittest",
+      ":psnr",
+    ]
+  }
+}
+
 static_library("libyuv") {
   sources = [
     # Headers
     "include/libyuv.h",
     "include/libyuv/basic_types.h",
     "include/libyuv/compare.h",
     "include/libyuv/convert.h",
     "include/libyuv/convert_argb.h",
     "include/libyuv/convert_from.h",
     "include/libyuv/convert_from_argb.h",
@@ skipping 234 matching lines @@
   }
 
   executable("cpuid") {
     sources = [
       # sources
       "util/cpuid.c"
     ]
     deps = [ ":libyuv" ]
   }
 }