Fix header inclusion order fix

This fixes a header inclusion order bug in czmq_prelude.h

If you're compiling on linux with gcc -std=c99 _AND_
you are including czmq.h _after_ stdint.h _and_
you are not defining _BSD_SOURCE _before_ stdint.h,
uint is not properly defined, causing a build error.
This patch fixes that error. It also fixes the feature detect
code for Linux hosts.