Add OpenBSD Support

[LainLayer]

Minor nob modifications to support compiling on OpenBSD.
Hot reloading seems to work, so does rendering.

The audio when playing an audio file sounds very slowed down, too bad, I don't know how to fix it. Everything else works :)

[LainLayer]

It seems the messed up audio is a general Raylib/miniaudio phenomena on OpenBSD and not a fault in the musializer code. Considering no one cares about OpenBSD, it will likely take a while until it is fixed. Until then it at least compiles and runs.

[ismagilli]

On FreeBSD dl* functions live in libc. On linux ld* functions placed in a separate library (libdl). I think the situation with OpenBSD is similar to the situation with FreeBSD. Possible solution: ln -s /usr/lib/libc.so /usr/lib/libdl.so as root. @LainLayer It seems to me that it is better to use the same implementation of nob on linux and BSD, put the -ldl flag in a separate call to the nob_cmd_append function and wrap it in ifdef-endif. And maybe rename target from LINUX to UNIX.

BSD detection (see boost/bsd.hpp):

#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || defined(__DragonFly__)
// Your code here...
#endif

[ismagilli]

Note that this is only MY opinion. It is better to wait rexim reply.

[rexim]

@ismagilli I was thinking about making a more general POSIX builder sort of speak. But I'll do it myself later.

Thank you for the contribution guys!