Fix build with system yasm.

libvpx.gyp

 # Copyright (c) 2011 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': {
     # Generic libvpx C sources for both x86 and ARM targets.
     'libvpx_generic_sources': [
       'source/libvpx/vp8/common/alloccommon.c',
       'source/libvpx/vp8/common/blockd.c',
       'source/libvpx/vp8/common/debugmodes.c',
@@ skipping 78 matching lines @@
     [ '(OS=="linux" or OS=="mac" or OS=="win") and target_arch!="arm"', {
       'targets': [
         {
           # This libvpx target contains both encoder and decoder.
           # Encoder is configured to be realtime only.
           'target_name': 'libvpx',
           'type': 'static_library',
           'variables': {
             'shared_generated_dir':
               '<(SHARED_INTERMEDIATE_DIR)/third_party/libvpx',
-            'yasm_path': '<(PRODUCT_DIR)/yasm',
             'conditions': [
               [ 'OS=="linux" and target_arch=="ia32"', {
                 'yasm_flags': [
                   '-felf32',
                   '-m', 'x86',
                   '-I', 'source/config/linux/ia32',
                   '-I', 'source/libvpx',
                 ],
               }],
               [ 'OS=="linux" and target_arch=="x64"', {
@@ skipping 14 matching lines @@
               }],
               [ 'OS=="win" and target_arch=="ia32"', {
                 'yasm_flags': [
                   '-DPREFIX',
                   '-fwin32',
                   '-m', 'x86',
                   '-I', 'source/config/win/ia32',
                   '-I', 'source/libvpx',
                 ],
               }],
+              [ 'use_system_yasm==0', {
+                'yasm_path': '<(PRODUCT_DIR)/yasm',
+              }, {  # use_system_yasm == 1
+                'yasm_path': '<!(which yasm)',
+              }],
             ],
           },
-          'dependencies': [
-            '../yasm/yasm.gyp:yasm#host',
+          'conditions': [
+            [ 'use_system_yasm==0', {
+              'dependencies': [
+                '../yasm/yasm.gyp:yasm#host',
+              ],
+            }],
           ],
           'include_dirs': [
             'source/config/<(OS)/<(target_arch)',
             'source/libvpx',
             'source/libvpx/vp8/common',
             'source/libvpx/vp8/decoder',
             'source/libvpx/vp8/encoder',
           ],
           'direct_dependent_settings': {
             'include_dirs': [
@@ skipping 371 matching lines @@
       },
     },
   ],
 }
 
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