Fix build with system yasm.

third_party/yasm/yasm_compile.gypi

 # 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.
 
 # This is an gyp include to use YASM for compiling assembly files.
 #
 # Files to be compiled with YASM should have an extension of .asm.
 #
 # There are two variables for this include:
 # yasm_flags : Pass additional flags into YASM.
 # yasm_output_path : Output directory for the compiled object files.
 #
 # Sample usage:
 # 'sources': [
 #   'ultra_optimized_awesome.asm',
 # ],
 # 'variables': {
 #   'yasm_flags': [
 #     '-I', 'assembly_include',
 #   ],
 #   'yasm_output_path': '<(SHARED_INTERMEDIATE_DIR)/project',
 # },
 # 'includes': [
 #   'third_party/yasm/yasm_compile.gypi'
 # ],
 
 {
   'variables': {
     'yasm_flags': [],
-    'yasm_path': '<(PRODUCT_DIR)/yasm',
 
     'conditions': [
+      [ 'use_system_yasm==0', {
+        'yasm_path': '<(PRODUCT_DIR)/yasm',
+      }, {
+        'yasm_path': '<!(which yasm)',
+      }],
+
       # Define yasm_flags that pass into YASM.
       [ 'OS=="linux" and target_arch=="ia32"', {
         'yasm_flags': [
           '-felf32',
           '-m', 'x86',
         ],
       }],
       [ 'OS=="linux" and target_arch=="x64"', {
         'yasm_flags': [
           '-DPIC',
@@ skipping 20 matching lines @@
         'asm_obj_extension': 'obj',
       }, {
         'asm_obj_extension': 'o',
       }],
     ],
   },  # variables
 
   'conditions': [
     # Only depend on YASM on x86 systems, do this so that compiling
     # .asm files for ARM will fail.
-    ['target_arch=="ia32" or target_arch=="x64"', {
+    ['use_system_yasm==0 and ( target_arch=="ia32" or target_arch=="x64" )', {
       'dependencies': [
         '<(DEPTH)/third_party/yasm/yasm.gyp:yasm#host',
       ],
     }],
   ],  # conditions
 
   'rules': [
     {
       'rule_name': 'assemble',
       'extension': 'asm',
       'inputs': [ '<(yasm_path)', ],
       'outputs': [
         '<(yasm_output_path)/<(RULE_INPUT_ROOT).<(asm_obj_extension)',
       ],
       'action': [
         '<(yasm_path)',
         '<@(yasm_flags)',
         '-o', '<(yasm_output_path)/<(RULE_INPUT_ROOT).<(asm_obj_extension)',
         '<(RULE_INPUT_PATH)',
       ],
       'process_outputs_as_sources': 1,
       'message': 'Compile assemly <(RULE_INPUT_PATH).',
     },
   ],  # rules
 }