getopt: remove hack for special handling of "-" argument

getopt: remove hack for special handling of "-" argument

Older versions of Getopt::Long acted bogusly and died when they
where configured with the 'bundling' flag and an argument '-' was
seen on the command line they were parsing.  That is no longer
the case though, and has not been for quite a long time: the bug
is no longer present in the 5.6.2 version of perl and the 2.25
version of Getopt::Long (and today, the latest versions of perl
and Getopt::Long are respectively 5.14.2 and 2.38).  The obsolete
workaround for that Getopt::Long bug can thus be removed from our
'getopt' function.

It is also worth noting that such a workaround was quite buggy
and brittle itself; for example, a command like this:
  "autom4te --output -"
would have caused the incorrect diagnostic:
  "autom4te: option `--output' requires an argument"
Much worse, a command like this:
  "autom4te --language=autoconf --output - configure.ac"
would have caused the standard input of autom4te to be processed
and copied into the 'configure.ac' file, deleting its pre-existing
content!  Surely not what a user would have expected.

After this change, a command like this:
  autom4te --language=autoconf --output - - <configure.ac >out
works as expected, processing the input from 'configure.ac' and
writing it to the 'out' file.

* lib/Autom4te/General.pm (use): Require perl version 5.6.2.
(getopt): Remove the old workaround.