devel platforms without autoconf 2.53

Who here is using a development platform that does not have *easy*
access to autoconf 2.53?

I'm asking because it would be nice to fully flush the autoconf-25
branch to HEAD, but only if it won't cause trouble for anyone.

For distributors I don't see this causing any difficulty, because squid
would not be the first package that requires autoconf 2.53, thus they
already have the infrastructure in place.

For end users building from source but not hacking, this doesn't affect
them due to the use of AM_MAINTAINER_MODE.

Rob