Work around DJGPP shell function return bug with command substitutions.

Work around DJGPP shell function return bug with command substitutions.

DJGPP bash 2.04 has a bug in that `return $ac_retval' done in a
shell function which also contains a command substitution causes
the shell to barf.  For more details and a fix see:
<http://www.delorie.com/djgpp/mail-archives/browse.cgi?p=djgpp-workers/2009/09/09/03:35:08>
Possible workaround include putting the `return' in a subshell
or calling another function to set the status.

* lib/autoconf/general.m4 (_AC_PREPROC_IFELSE_BODY)
(_AC_COMPILE_IFELSE_BODY, _AC_LINK_IFELSE_BODY)
(_AC_RUN_IFELSE_BODY, _AC_COMPUTE_INT_BODY): Use AS_SET_STATUS
instead of `return'.
* doc/autoconf.texi (Common Shell Constructs, Shell Functions):
Document the issue.
* THANKS: Update.
Report by Rugxulo and Reuben Thomas.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>