_AS_ECHO_N_PREPARE - bug correction in ksh88 on AIX 6.1

[Jan Madzik]

Hi,
I found bug in korn shell (version M-11/16/88f) on AIX 6.1 (tested
only on one server instance) and due to this bug I couldn't correctly
configure ImageMagick.

> uname -a
AIX myhost 1 6 00CB25304C00

Here is minimal script that shows this bug:
#!/bin/sh
echo `echo 'hello bug\c'`
echo `echo this text fall to blackhole`
echo `echo next line`

and output:
h
hello bug
next line

Generated configure and config.status scripts contains some tests that
causes wrong initialization of some variables later.

Fragments from configure script:
...
...
ECHO_C= ECHO_N= ECHO_T=
case `echo -n x` in
-n*)
 case `echo 'x\c'` in
 *c*) ECHO_T=' ';;     # ECHO_T is single tab character.
 *)   ECHO_C='\c';;
 esac;;
*)
 ECHO_N='-n';;
esac
...
...
...
ac_prefix_conf_OUT=`echo magick/magick-config.h`
...
...

Variable ac_prefix_conf_OUT contains only one character - x.

My solution was to configure it with another shell (./configure
SHELL=/bin/ksh93) but I think it would be nice to handle this bug
in autoconf.

I'm not sure if it is ok, but this patch helps me:

diff --git a/lib/m4sugar/m4sh.m4 b/lib/m4sugar/m4sh.m4
index c7b0bff..b884a88 100644
--- a/lib/m4sugar/m4sh.m4
+++ b/lib/m4sugar/m4sh.m4
@@ -785,9 +785,11 @@ m4_defun([_AS_ECHO_N_PREPARE],
 [ECHO_C= ECHO_N= ECHO_T=
 case `echo -n x` in @%:@((((
 -n*)
-  case `echo 'x\c'` in
+  case `echo 'xy\c'` in
  *c*) ECHO_T='        ';;     # ECHO_T is single tab character.
-  *)   ECHO_C='\c';;
+  xy*) ECHO_C='\c';;
+  *)   echo `echo bug ksh88 AIX 6.1` > /dev/null       # this line
handles bug in ksh version M-11/16/88f on AIX 6.1
+       ECHO_C='\c' ;;
  esac;;
 *)
  ECHO_N='-n';;

But this patch solve just this particular test. It does not solve
further use of echo with ECHO_C variable in command substitution.