functions: improve cross-compilation guesses for glibc system hosts

functions: improve cross-compilation guesses for glibc system hosts

Paul and Eric suggested that improving the cross-compilation guesses
for targets that are glibc systems would be welcome here [1][2].

This patch modifies the cross-compilation behaviour of
  AC_FUNC_CHOWN
  AC_FUNC_GETGROUPS
  AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK
  AC_FUNC_MALLOC
  AC_FUNC_MMAP
  AC_FUNC_REALLOC
  AC_FUNC_STRCOLL
so that when the target is a glibc system (or, in case of AC_FUNC_MALLOC
and AC_FUNC_REALLOC, any known "good" Unix system), the guess is
"yes it works" rather than "guessing no".

This is important because some of these macros are used in Gnulib, and
in case of "guessing no" Gnulib provides extra workaround code, and
  1) Generally, when targetting embedded systems, code size should be
     minimized,
  2) In [3], unnecessary workaround code will look like a Glibc bug.

This patch also changes the configure output to "guessing yes" or
"guessing no" in a case where the ac_cv_* variable is undocumented.

[1] http://lists.gnu.org/archive/html/bug-gnulib/2012-05/msg00010.html
[2] http://lists.gnu.org/archive/html/bug-gnulib/2012-05/msg00013.html
[3] http://sourceware.org/glibc/wiki/Testing/Gnulib

This patch was tested with the simple configure.ac file
=============== configure.ac ==================
AC_INIT([dummy], [0])
AC_FUNC_CHOWN
AC_FUNC_GETGROUPS
AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK
AC_FUNC_MALLOC
AC_FUNC_MMAP
AC_FUNC_REALLOC
AC_FUNC_STRCOLL
AC_OUTPUT
===============================================

* lib/autoconf/functions.m4 (AC_FUNC_CHOWN): Require AC_CANONICAL_HOST.
When cross-compiling to a glibc system, guess yes.
(AC_FUNC_GETGROUPS): Likewise.
(AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK): Likewise.
(AC_FUNC_STRCOLL): Likewise.
(_AC_FUNC_MALLOC_IF): Require AC_CANONICAL_HOST. When cross-compiling
to a know Unix system other than AIX and OSF/1, guess yes.
(_AC_FUNC_REALLOC_IF): Likewise.
(AC_FUNC_MMAP): Require AC_CANONICAL_HOST. When cross-compiling to a
system with a Linux kernel, guess yes.