maint: require perl 5.6 throughout

maint: require perl 5.6 throughout

That version of perl is quite old already, so it should be OK to
require it.  More importantly, it is the older perl versions the
developers can currently use for testing, thus it's safer and more
honest to just require it throughout.  And anyway, we were already
requiring it in Automake::Getopt, which implied that both automake
and aclocal wouldn't have worked in practice with an older perl
version.

See also the discussion related to automake bug#10925.

* lib/Automake/ChannelDefs.pm: Adjusted to require perl 5.6.
* lib/Automake/Channels.pm: Likewise.
* lib/Automake/Condition.pm: Likewise.
* lib/Automake/Configure_ac.pm: Likewise.
* lib/Automake/DisjConditions.pm: Likewise.
* lib/Automake/FileUtils.pm: Likewise.
* lib/Automake/General.pm: Likewise.
* lib/Automake/Item.pm: Likewise.
* lib/Automake/ItemDef.pm: Likewise.
* lib/Automake/Location.pm: Likewise.
* lib/Automake/Options.pm: Likewise.
* lib/Automake/Rule.pm: Likewise.
* lib/Automake/RuleDef.pm: Likewise.
* lib/Automake/Struct.pm: Likewise.
* lib/Automake/VarDef.pm: Likewise.
* lib/Automake/Variable.pm: Likewise.
* lib/Automake/Version.pm: Likewise.
* lib/Automake/Wrap.pm: Likewise.
* lib/Automake/XFile.pm: Likewise.