argp: is now a standalone CMake project. Added its own public include path to its own build. Helps finding argp.h when compiling argp...
argp.h: removed inclusion of getopt.h. We don't have a public getopt.h, and it's not required to be in argp.h anyway.
Got argp-eexst.c to compile: only use sysexits.h and EX_USAGE if they're available according to config.h, use stdlib.h and EXIT_FAILURE otherwise.