On Linux, ldconfig needs to be run after installing shared libraries
to system paths (typically /usr/local/lib, /usr/lib and /lib).
Document this.

Fixes #586

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4420 d4fa192b-c00b-0410-8231-f00ffab90ce4