OSX Maveriks compiler error installing pip package

If you get this error:

$ pip install lmxl
...

clang: error: unknown argument: '-mno-fused-madd' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
error: command 'cc' failed with exit status 1

You can use this command to fix it (append to the beginning of the command):

$ ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future pip install lxml

Profit!