enable to configure with older versions of autoconf

https://github.com/joyent/libuv/issues/1158
(already closed)

configure.ac at master: AC_PREREQ(2.57)
This means that libuv is enable to configure with autoconf version 2.57 or newer.

But AS_CASE is enable to use autoconf 2.60 or newer version.And AS_CASE has a BUG until version 2.69.
So AC_PREREQ(2.69) might be more better solution.

If you can support older versions of autoconf, please check this patch.
Regards and great thanks for your very useful software.