Fix build with system yasm.

third_party/yasm/yasm_compile.gypi

Index: third_party/yasm/yasm_compile.gypi
===================================================================
--- third_party/yasm/yasm_compile.gypi	(revision 105085)
+++ third_party/yasm/yasm_compile.gypi	(working copy)
@@ -27,9 +27,14 @@
 {
   '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': [
@@ -70,7 +75,7 @@
   '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',
       ],