OSX support

These changes allowed me to successfully run under OSX 10.9.1. 

I split them in two commits because the makefile is a bit shit, tbh, so you might want to review what's happening in that one. Note that I started from your makefile.unix and copied it to be the new makefile.osx, leaving the old one as makefile.osx-bitcoin -- so you want to diff makefile.unix with .osx to see what I've done.
In particular, I had to remove "LDHARDENING+=-Wl,-z,relro -Wl,-z,now" because ld -z does not exist in OSX. I hope that's not a terrible security bug.

The other changes ( #83dcd04 ) seem small enough and I think they could go in master.

I'll post a writeup of other things one needs (i.e. defining all necessary variables to match a local environment) later on.