Disable -Wunknown-warning-option in HarfBuzz port

Author: kleisauke

tools/ports/harfbuzz.py

@@ -132,6 +132,8 @@ def create(final):
 
     # https://github.com/harfbuzz/harfbuzz/commit/60c6b7786d9f4651ae2803bfc4ff4435b38a5bc6
     cflags.append('-Wno-error=cast-function-type-strict')
+    # TODO(kleisauke): Remove when LLVM rolls in
+    cflags.append('-Wno=error=unknown-warning-option')
 
     ports.build_port(os.path.join(source_path, 'src'), final, 'harfbuzz', flags=cflags, srcs=srcs)