Fix PyQt build script. Fixes #33576 to allow bottled PyQt again

@mikemcquaid This is the proper fix for #27623 and #33576. Verified using:

```
brew install pyqt-4.11.1.mavericks.bottle.1.tar.gz 
==> Pouring pyqt-4.11.1.mavericks.bottle.1.tar.gz
==> Caveats
Phonon support is broken.
==> Summary
🍺  /usr/local/Cellar/pyqt/4.11.1: 560 files, 19M
```
Checked that files exist in /usr/local/share/sip and finally checked with installing pyqwt.

What would I need to do to propose bottles for pyqt and pyqwt?
`mkdir_p` won't error if it already exists, I believe.