configure.in: enable silet rules by default

Makes the compilation output more graspable.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
configure.ac: rename from configure.in

The configure script has a preferred extension of .ac, so rename it to
adjust with times.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>