Use getopt if it is available instead of sc_getopt

Since CMake is already looking for getopt, I thought it made sense to use it if it is available. So I added some logic to use getopt if CMake finds it. While I was doing that, I noticed that check-express would crash if run without arguments. The problem is that in fedex.c there is one call to ERRORusage_function that doesn't include a check to make sure that ERRORusage_function actually points to something. Maybe I should have done separate pull requests since I'm really changing two different things, but figured this was easiest.