- Improved gcc detection based on experience on Ubuntu.

- Reenabled -Wno-unused-but-set-variable compiler flag for gcc builds.

- Fixed library ordering for example program builds.
Fixed libuinet library dependencies for Linux.
Fixed up sysctl api routine parameter names for consistency and
avoidance of compiler error shenanigans when building with g++.
Fix up net/if.h vs. linux/if.h include issues on Linux such that
current netmap code is happy.
Expose required types under Linux.
Cleanup sample programs, removing unneeded (and nonportable) pthread_t
initializations and updating calls to uinet_interface_up().