cmake: unbreak system libusb support

Reference libusb doesn't support DragonFly and FreeBSD because those
ship a different libusb implementation (supports 0.1, 1.0, 2.0 API).

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(usbi_alloc_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(usbi_alloc_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_has_capability) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_has_capability) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_has_capability) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(usbi_connect_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(usbi_disconnect_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_get_device_list) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_get_device_list) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_unref_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_unref_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_write
>>> referenced by core.c
>>>               core.c.o:(usbi_signal_event) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_read
>>> referenced by core.c
>>>               core.c.o:(usbi_clear_event) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_wrap_sys_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_wrap_sys_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_wrap_sys_device) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_open) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_backend
>>> referenced by core.c
>>>               core.c.o:(libusb_open) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_read
>>> referenced by core.c
>>>               core.c.o:(libusb_close) in archive externals/libusb/libusb.a

ld: error: undefined symbol: usbi_write
>>> referenced by core.c
>>>               core.c.o:(libusb_close) in archive externals/libusb/libusb.a

ld: error: too many errors emitted, stopping now (use -error-limit=0 to see all errors)
c++: error: linker command failed with exit code 1 (use -v to see invocation)