Fix build script for C examples

Should use the same library (`libzmq`) that queries `pkg-config` for,
and not a different one (`libczmq`).
Include time.h and sys/time.h on Linux systems

Both library files are needed for included symbols (nanosleep() and
gettimeofday()) in Linux and BSD systems.

Furthermore, windows.h is needed to in Windows to access Win32 API, like
Sleep() and GetSystemTime().  So, this is the library file we should
include under Windows compilation.
Merge pull request #458 from xakon/master

Fix build system in C examples