Enable building on FreeBSD hosts

[porsolic]

Here is cmake build system patched to build blisp on FreeBSD host.
FreeBSD has libusb built in (/usr/lib/libusb.*) and libserial can be installed as package (pkg install libserialport) and then files /usr/local/include/libserialport.h and /usr/local/lib/libserialport.* will be available.

On my system blisp tool can be built without this line in CMakeLists.txt:

target_link_libraries(libblisp PRIVATE -L/usr/local/lib usb serialport)

but I included it anyway to be consistent with other platforms.

[robertlipe]

Do you have lubisb-config such that we can automate this without a list of special cases?

[porsolic]

There are:

/usr/libdata/pkgconfig/libusb-0.1.pc
/usr/libdata/pkgconfig/libusb-1.0.pc
/usr/libdata/pkgconfig/libusb-2.0.pc
/usr/local/libdata/pkgconfig/libserialport.pc

[gamelaster]

Thanks!