check for max cmd line length

[Marcus Brinkmann]

Hi,

the check for the maximum command line length is a bit harmful
on the Hurd, because the Hurd has no limit.  From reading the code,
it seems there is a safety check to bail out at 1 MB, but it is
excessive to check for that every time ltconfig runs.

Please don't check for this on the Hurd, (*-gnu*), but just set it
to any maximum value you need.

Thanks,
Marcus