setup.py creates a zombie C extension called "freetype2"

Since #3067, there has been a useless and unimportable extension created as part of the matplotlib install.  We need to remove it.

However, it reportedly fixes a real bug for some, but it's not quite clear how.  I'll poke around and see if I can break/fix it in the same way as the original reports and then find a better solution that doesn't install a broken `.so` file.

```
>>> import freetype2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: dynamic module does not define init function (initfreetype2)
```

The name of matplotlib's freetype wrapper is ft2font, not freetype2.
Yes, it's the crux of the fix here. At least on my system, the result from `freetype-config --cflags` is totally and utterly wrong (`-I/freetype2`), so better to use pkg-config and, failing that, fall back to the regular default locations.  I suspect this was the source of all of the problems that led to the #3067 solution in the first place.