Get godot to build on FreeBSD

There were a few linuxisms that were preventing godot from compiling on FreeBSD.  These changes allow it to build.  I've tested building on Linux and FreeBSD and bin/godot does start up and seem usable.

The joystick code is not working as I'm not familiar with the FreeBSD joystick API.  I ifdefed out the body of the functions. I realize this is probably frowned upon (I don't do much C/C++ stuff) so I'd appreciate a suggestion for a cleaner way to do that.  SDL looks like it might be a good model for cross-platform joystick support.