use of which in common/autoconf/autogen.sh is not portable

This was introduced by the fix for bug #JDK-8009988 "build-infra: Fix configure output for zip debuginfo check"
http://bugs.sun.com/view_bug.do?bug_id=8009988

Changeset 15c1642967c9 added use of the which command in common/autoconf/autogen.sh.

%cat -n common/autoconf/autogen.sh
     [...]
    46 AUTOCONF=$(which autoconf 2> /dev/null);
    47 AUTOCONF_267=$(which autoconf-2.67 2> /dev/null);
    48
    49 echo "Autoconf found: ${AUTOCONF}"
    50 echo "Autoconf-2.67 found: ${AUTOCONF_267}"
    51
    52 if test "x${AUTOCONF}" = x; then
    53 echo You need autoconf installed to be able to regenerate the configure script
    54 echo Error: Cannot find autoconf 1>&2
    55 exit 1
    56 fi
    57
    58 if test "x${AUTOCONF_267}" != x; then
    59 AUTOCONF=${AUTOCONF_267};
    60 fi

If there is any value at all in AUTOCONF_267, that is what the script will try to use at line 59.

This fails on Solaris 11 x64 where the variable AUTOCONF_267 is set to a value of 'no autoconf-2.67 in /opt/java/jdk1.6.0/bin /opt/devtools/i586/SUNWspro/SS12u1/bin /w/tbell/bin /usr/sbin /usr/bin /usr/local/bin /usr/bin /usr/sfw/bin /usr/gnu/bin /usr/X11/bin /usr/ccs/bin /usr/dt/bin'

The result is not good: the generated-configure.sh files created are zero bytes long.