Extra steps needed to build on Mac OS

[rchoudhary]

To get this program to build on Mac OS 10.13 (High Sierra) I needed to do the following steps:

1. Install argp-standalone using Homebrew like so:
   brew install argp-standalone
2. Add -I/usr/local/include/ to CFLAGS and -largp to LDFLAGS in the Makefile
3. Add #include <limits.h> to the top of argparser.c

I believe that Step 1 should be mentioned in the README, and that Steps 2 and 3 should be changed in the Makefile and argparser.c respectively.

[NewLunarFire]

I agree. If you could submit a PR I will review it. Those steps aren't necessary on Linux, so I will make sure this still builds on Linux before merging.