config: update AC_PREREQ statement to 2.60

Unrelated to the previous patches, the new value simply reflects
the reality that the minimum level for autoconf to configure
all x.org modules is 2.60 dated June 2006.

ftp://ftp.gnu.org/gnu/autoconf/autoconf-2.60.tar.gz



Signed-off-by: Gaetan Nadon <memsize@videotron.ca>