squid: fix automake breakage

Squid bundles a copy of libltdl (from libtool) which autoreconfigures on
its own.
For some odd reason when automake was bumped to version 1.15 and if the host
system has another automake version, for example 1.14, the ACLOCAL and
AUTOMAKE variables don't expand properly when the internal autoreconf is
triggered hence calling the missing handler which in turn tries to use
an incorrect automake version.
The solution is to pass unexpanded ACLOCAL and AUTOMAKE variables that
defer the evaluation to a later moment and avoid the issue.

Fixes:
http://autobuild.buildroot.net/results/73f/73fcffafbea320f8c64378bbe8a96922b5e7c6b5/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>