Add empty files to empty targets to appease XCode.

gyp/opts.gypi

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
         'none_sources': [
             '<(skia_src_path)/opts/SkBitmapProcState_opts_none.cpp',
             '<(skia_src_path)/opts/SkBlitMask_opts_none.cpp',
             '<(skia_src_path)/opts/SkBlitRow_opts_none.cpp',
         ],
@@ skipping 34 matching lines @@
             '<(skia_src_path)/opts/opts_check_x86.cpp',
         ],
         'ssse3_sources': [
             '<(skia_src_path)/opts/SkBitmapProcState_opts_SSSE3.cpp',
             '<(skia_src_path)/opts/SkOpts_ssse3.cpp',
         ],
         'sse41_sources': [
             '<(skia_src_path)/opts/SkBlitRow_opts_SSE4.cpp',
             '<(skia_src_path)/opts/SkOpts_sse41.cpp',
         ],
+        # These targets are empty, but XCode doesn't like that, so add an empty file to each.
         'sse42_sources': [
+            '<(skia_src_path)/core/SkForceCPlusPlusLinking.cpp',
         ],
         'avx_sources': [
+            '<(skia_src_path)/core/SkForceCPlusPlusLinking.cpp',
         ],
         'avx2_sources': [
+            '<(skia_src_path)/core/SkForceCPlusPlusLinking.cpp',
         ],
 }