Fix build on some 64bit platforms.
Says David Faure:
> SVN commit 657301 by dfaure:
>
> Fix libtool hardcoding /usr/lib RPATH everywhere on kubuntu-amd64,
> making it impossible to use my own Qt. libtool doesn't add rpaths for
> the paths in $sys_lib_dlsearch_path_spec, but that var was set to
> "/lib64 /usr/lib64", due to /lib64 existing (but as a symlink to
> /lib). -> only use /lib64 instead of /lib in
> sys_lib_dlsearch_path_spec if /lib64 is a real dir, not if it's a
> symlink. Die libtool, die.
Noticed by André Wöbbeking.