* lib/autoconf/general.m4 (_AC_INIT_DEFAULTS): Redirect stdin from

* lib/autoconf/general.m4 (_AC_INIT_DEFAULTS): Redirect stdin from
/dev/null, as "configure" shouldn't read stdin, and this insulates
us from problems (e.g., when testing for "cl").  Also, do this
redirection before invoking "hostname" or "uname", and keep the
original input stream available via...
(AS_ORIGINAL_STDIN_FD): ... this new macro.
(_AC_PREPROC_IFELSE, _AC_COMPILE_IFELSE, _AC_LINK_IFELSE): Don't
bother with "</dev/null" since it's now done at the top of
'configure'.
* lib/autoconf/c.m4 (AC_PROG_CC, AC_PROG_CXX): Likewise.
* lib/autoconf/fortran.m4 (_AC_PROG_FC): Likewise.
* lib/autoconf/programs.m4 (_AC_PATH_PROG_FLAVOR_GNU): Likewise.
* doc/autoconf.texi (File Descriptor Macros): New section.
(Printing Messages): Mention it.
* tests/base.at (Input/Output): New test.