tar: update ancient configure cruft

* configure.ac: Use AC_PROG_CC_STDC, not just AC_PROG_CC.
Remove obsolete macros AC_ISC_POSIX, AC_HEADER_SYS_WAIT,
AC_HEADER_DIRENT, AC_HEADER_STAT, AC_HEADER_STDC, AC_TYPE_SIGNAL,
AC_TYPE_SIZE_T.
* lib/prepargs.c (IN_CTYPE_DOMAIN): Remove.  All uses removed.
* src/list.c (from_header): Use isspace, not ISSPACE.
* src/system.c (pipe_handler, sys_exec_info_script):
* src/tar.c (sigstat):
Use void, not RETSIGTYPE.