disable unused argument warning for clang builds.

normally this warning is disabled but for nacl builds that
use clang its not.  this CL makes the option universally disabled
for all clang based builds.

BUG=libyuv:581
TESTED=native_client/build/gyp_nacl all.gyp -Dgtest_target_type=executable -Dmsan=0 & ninja -C out/Debug libyuv
R=kjellander@chromium.org

Review URL: https://codereview.chromium.org/1865183002 .

libyuv.gyp

@@ -47,6 +47,12 @@
       # Allows libyuv.a redistributable library without external dependencies.
       'standalone_static_library': 1,
       'conditions': [
+       # Disable -Wunused-parameter
+        ['clang == 1', {
+          'cflags': [
+            '-Wno-unused-parameter',
+         ],
+        }],
         ['build_neon != 0', {
           'defines': [
             'LIBYUV_NEON',