This will let people with Macintosh OS/X computers compile and use stlink with STLink/V2 devices, as libsg3 is not available for that platform.
Compiles, but not fully tested yet.
Move towards a standard libstlink, with backends for libusb (stm32l discovery) and scsi passthrough (stm32vl discovery) and a common front end. Verified that stm32l branch works much the same, but more to go for stm32vl.