Added darwin support

[twhiston]

Hi,

I've added some functions to your port to get it to compile on Darwin (OSX) systems. Mainly this just involved creating a few functions you get in Linux and changing a few printf's to use the right types. All of these changes are wrapped in an #ifdef which is only defined if __APPLE__ or __MACH__ are defined by the system. I also added an UNUSED macro which you can wrap unused vars in to deal with compiler errors.

The one 'annoying' change is that clang doesn't like -pthread in the linker flags, so i had to remove this for darwin builds. Rather than put the logic to deal with this in to every demo makefile i broke it out into an include and added this file to all the makefiles instead.

After doing these things build_all.sh works perfectly on osx and all the demo programs build and run.

[twhiston]

@michaelbecker would love to get your thoughts on this PR. Thanks for all the great work on this repo!

[abeck70]

@michaelbecker i would be keen to see os x support and can help test this.