vici: Make installation of Ruby Gem and Python Egg optional

vici: Make installation of Ruby Gem and Python Egg optional

Installing them might not work well when building distro packages (e.g.
with DESTDIR installs).  It might be easier to install them later with a
script in the distro package.

When building from source on the local system it could still be useful to
install the packages directly, which can be enabled with separate configure
options.

The main problem with DESTDIR installations of the Python Egg is that
easy_install creates or modifies a file called easy-install.pth in the
installation directory.  So it's not actually possible to simply copy
the results in DESTDIR over to the actual system as that file would have
to be merged with any existing one.

Fixes #914.