Fix build with system yasm.

libvpx.gyp

Index: libvpx.gyp
===================================================================
--- libvpx.gyp	(revision 103347)
+++ libvpx.gyp	(working copy)
@@ -96,7 +96,6 @@
           'variables': {
             'shared_generated_dir':
               '<(SHARED_INTERMEDIATE_DIR)/third_party/libvpx',
-            'yasm_path': '<(PRODUCT_DIR)/yasm',
             'conditions': [
               [ 'OS=="linux" and target_arch=="ia32"', {
                 'yasm_flags': [
@@ -131,10 +130,19 @@
                   '-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)',