Rename libvpx_untrusted->libvpx_nacl. Disable non-portable NaCl build.

libvpx_nacl.gyp

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'variables': {
     'libvpx_source%': 'source/libvpx',
     'werror': '',
   },
   'includes': [
     '../../native_client/build/untrusted.gypi',
   ],
   'targets': [
     {
-      'target_name': 'libvpx_untrusted',
+      'target_name': 'libvpx_nacl',
       'type': 'none',
       'variables': {
-        'nlib_target': 'libvpx_untrusted.a',
-        'build_glibc': 1,
-        'build_newlib': 1,
+        'nlib_target': 'libvpx_nacl.a',
+        'build_glibc': 0,
+        'build_newlib': 0,
         'build_pnacl_newlib': 1,
       },
       'dependencies': [
         '<(DEPTH)/native_client/tools.gyp:prep_toolchain',
       ],
       'include_dirs': [
         'source/config/nacl',
         'source/config',
         '<(libvpx_source)',
         '<(libvpx_source)/vp8/common',
         '<(libvpx_source)/vp8/decoder',
         '<(libvpx_source)/vp8/encoder',
       ],
       'includes': ['libvpx_srcs_nacl.gypi', ],
-    },  # end of target 'libvpx_untrusted'
+    },  # end of target 'libvpx_nacl'
   ],
 }