Fixed compliation for OpenBSD 7.0

[Crest]

On OpenBSD libusb is provided by the libusb1 port and the existing Findlibusb.cmake module failed to detect it properly because it installs to /usr/local in accordance with the OpenBSD filesystem hierarchy. Teaching cmake the OpenBSD file system layout gets us half the way.

The second and last problem I've encountered is that the MINIMAL_API_VERSION required under OpenBSD is undeclared causing a compile error in libusb_settings.h. I used libusb 1.0.23p2 installed from the official binary package repository, but I don't see any reasons why OpenBSD should require a newer libusb version than other platforms.

[nbonfils]

I hope it's not to out of topic, but I've been so frustrated with AVR related things on OpenBSD, the fact that stlink built out of the box without needing to touch anything was so relieving. Literally made my whole weekend better!
I take it's thanks to @Crest and also to the stlink-org team (@Nightwalker-87 and @xor-gate it seems) to have made a software that's not unfriendly to the "fringe" OSes.
Thank you <3

[Nightwalker-87]

@nbonfils Thank you very much. It is good to hear such feedback from affected users.
It is indeed a key objective of this toolset to maintain best possible support for a wide range of operating environments (as a far as we can sustain compatibility regarding libraries and packages). As it appears we are on a good way here... 😃

May I include your review on our main page to give additional orientation to new users? Please let me know.

[nbonfils]

Of course @Nightwalker-87, feel free to do so.
This key objective definitely deserves more appreciation. It might even be worth mentioning in the README as this sort of details usually hints users like me that this software and the maintainers care. We totally need more of that.
Thank you again.