OS X Mavericks introduced a warning in Clang that causes builds to fail because

of incorrect arguments to `cc`. The common workaround for this is to use the

ARCHFLAGS environment variable to specify an additional -Wno-error in order to

turn this back into something that doesn't fail, but the P4Python installer

overrides ARCHFLAGS, so instead we override CFLAGS.

Installed from setup.py successfully for the first time since upgrading to

Mavericks.