May need AC_PREREQ([2.60]) in configure.in

[Rhys Ulerich]

Hi,

While trying to build screen from the latest git sources on a slightly
older system, I noticed AC_USE_SYSTEM_EXTENSIONS appears in
http://git.savannah.gnu.org/cgit/screen.git/tree/src/configure.in.
The macro caused autogen.sh to bomb on a system with an pre-2.60
autoconf.  AC_PREREQ may need to be added to help those with too old
an autoconf figure it out a bit more quickly.

- Rhys