merge: configure.ac improvements

 * remove check on unix.h
 * remove check on stdarg.h
 * remove check on inline and const
 * remove check on wide chars
 * remove check on fgetpos
 * remove check on memmove
 * remove check on strerror
 * remove check on srand48
 * remove check on long long int
 * signal: remove test on SYS_SIGLIST_DECLARED
 * simplify sig_atomic_t check
 * remove check for <dirent.h> d_ino
 * drop LANGINFO checks
 * remove stuff defined in POSIX:2001
 * use basic getopt from unistd.h
 * drop HAVE_UNISTD_H
 * unused prototypes - unsupported systems
 * sys/types.h
 * sys/select.h
 * inttypes.h includes stdint.h
 * Drop obsolete iconv check
 * Drop another obsolete iconv check
 * tidy md5