Bug 1306640 - Silent a -Wunused-but-set-variable in cairo. We don't want to diverge too much from upstream r=lsalzman

MozReview-Commit-ID: jzPlpGZh76

--HG--
extra : rebase_source : 86d12bd2b91cbdcefbdfb78f7e17c9dfad273cf5

gfx/cairo/cairo/src/moz.build

@@ -222,6 +222,7 @@ if CONFIG['GNU_CC'] or CONFIG['CLANG_CL']:
         '-Wno-type-limits',
         '-Wno-missing-field-initializers',
         '-Wno-conversion',
+        '-Wno-unused-but-set-variable',
     ]
 if CONFIG['CLANG_CXX'] or CONFIG['CLANG_CL']:
     CFLAGS += [