Lock down version of autoconf

Currently three different versions of autoconf are commonly used to update generated-configure.sh, 2.67, 2.68 and 2.69. We should decide on a version and stick to it. From discussion on the open list it seems locking it to 2.69 would be acceptable by everyone. The easiest way to achieve this is to simply change the AC_PREREQ macro to require this minimum version. Perhaps we should also make sure no newer version is used either.

By fixing this we can also remove the 2.67 workaround in autogen.sh that is there today.