Fixed warning about unused variable with MSVC (buf is not used when strerror_r is not available).
Added missing getopt.h and getopt_int.h frmo glibc. Prefixed their names with "argp-" to avoid name clashes with getopt headers that might be present on a particular system (such as Cygwin, which has getopt, but no argp)
Fixed remaining compilation problems in argp-help.c and argp-parse.c