Make P4PythonInstaller work on OS X Mavericks.
Review Request #5822 — Created May 14, 2014 and submitted
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