libc++: Suppress a compiler warning when building yasm.

BUG=358678
R=hclam@chromium.org

Review URL: https://codereview.chromium.org/983143004

Cr-Commit-Position: refs/heads/master@{#319825}

third_party/yasm/yasm.gyp

@@ -56,6 +56,10 @@
     'version_file': 'version.mac',  # Generated by genversion.
     'genmodule_source': 'genmodule_outfile.c',
   },
+  'target_defaults': {
+    # Silence warnings in libc++ builds (C code doesn't need this flag).
+    'ldflags!': [ '-stdlib=libc++', ],
+  },
   'targets': [
     {
       'target_name': 'yasm',