Modify build environment to work on FreeBSD.

Specifically, 1) add 'pre_hooks' to rebar.config to use "gmake" on
FreeBSD rather than "make", which runs the standard BSD "make",
2) ensure that "make" is ONLY run on Linux and Darwin and 3) modify the
c_src/Makefile to detect FreeBSD as well as Linux and use the
"-with_pic" flag in both cases.

Also, modern FreeBSD uses clang as the system C/C++ compiler and 0MQ
does not build with clang, so we set CXX to g++.  FreeBSD users will have
to install a version of gcc from ports and manually link
/usr/local/bin/g++ to /usr/local/bin/g++XX.