More {usb,pci}.ids related fixes
We're not:
- Properly setting {pci,usb}_ids_path vars, resulting in passing wrong {USB,PCI}_IDS vars via cflags;
- Defining {USB,PCI]_IDS only when needed;
- We're defining those regardless, which is a regression compared to the previous behaviour, when using autotools;
NOTE: Those are the last bits I'd like to have merged before cutting a 1.7.1 release.