* 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").
Suggested by Alexandre Duret-Lutz. Also, do this redirection
before invoking "hostname" or "uname".
(_AC_PREPROC_IFELSE, _AC_COMPILE_IFELSE,
_AC_LINK_IFELSE): Undo previous change, as it's no longer needed.
* lib/autoconf/c.m4 (AC_PROG_CC, AC_PROG_CXX): Don't bother with
"</dev/null" since it's now done at the top of 'configure'.
* lib/autoconf/fortran.m4 (_AC_PROG_FC): Likewise.
* lib/autoconf/programs.m4 (_AC_PATH_PROG_FLAVOR_GNU): Likewise.
Also, replace grep with shell pattern-matching, to save a process.