rules.mk: use long --family-suffix= option to avoid ambiguity with '-G'

[anthrotype]

This is a minor patch which doesn't change the result of the ttfautohint command.

It is just to allow me to use the noto-hinted repository to test that my Python wrapper for ttfautohint produces identical results as the original ttfautohint executable.

Apparently, with python's argparse module (used to parse command line options) the following string, which in the makefile rules for noto-hinted is meant to append the "-G" suffix to the family names:

$ ttfautohint -F -G ...

.. is interpreted by argparse as an -F option without its required positional argument (hence an error) and that is followed by the -G option (or --hinting-limit PPEM in its long form).

See https://bugs.python.org/issue9334

The workaround is to use the long form with the equal sign, e.g. --family-suffix="-G", which works in both python and ttfautohint native executable.

Another minor thing is the use of -p option with mkdir to prevent error message in case folder already exists.