Minimal changes to let users update the linux base and the linux X11 bits
in any order:

• add the X11 lib path to ld.so.conf in the linux base port
• (re)generate the ld.so.cache file in the linux base port too
• don't change the ld.so.conf in the linux X11 port

At deinstall time the linux base port may still complain about a changed
ld.so.cache file. A clean way to solve this would be to use ("@unexec" and
"@exec") in the plist. Since the plist is autogenerated this would need
some little magic in the plist generation or we have to switch to a static
plist. Delay the decission about how to handle this until we know when/how
to update to a more recent linux base port.