Speeding up our XML parsing required using the lxml Python module. On OS X that requires using ARCHFLAGS when installing.

ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future

Turns out not to be super easy to find this solution and, as this in future versions of clang may be a real problem, this is just a temporary solution - see the full explanation here thank you.