Use pkgconfig to find PCL configuration

This patch fixes PCL builds for me with PCL in `/usr/local`, OpenNI and libusb-1.0 not installed. Instead of hardcoding paths, it uses `pkg-config` (through [this library](https://pypi.python.org/pypi/pkgconfig)) to find out compiler/linker settings.

I also cleaned up the `setup.py` file a bit.