Various minor fixes for GNU Autotools stuff and some C warnings

As a fellow open source developer at Cisco, I say: congrats on releasing libfnr!

I have a few, small patches that may (or may not?) be of interest to you.  I fixed some minor GNU Autotools issues, made some forward-portability functionality, and fixed a few warnings I was getting when compiling fnr.c on my OS X machine.

Feel free to cherry pick these commits... and/or disregard them altogether.