[Rivet] Rivet linker flags

[David Grellscheid]

Hi Andy,

I just noticed that the libtool.m4 copy in Rivet is not the one you get
from the upstream libtool project, but is the Debian-specific version.
The difference is that Debian sets "link_all_deplibs=no" which is not
recommended by the libtool guys. Googling the flag shows long
discussions about the merits of no vs. unknown vs. yes, with no clear
outcome. But since YODA, ThePEG and Herwig all have the upstream
version, I was wondering if there was a specific reason to pick the
Debian one here? If not, I'll make it consistent with the others.

See you,

  David