eglibc: enforce use of autoconf-2.69

Since we updated autoconf in r42855 we also need to enforce its use
while building eglibc to allow it to build successfully.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43352 3c298f89-4303-0410-b956-a3cf2f4a3e73