if I do in a terminal:
$ file /usr/local/lib
also it is not a symbolic link.
However I do not have an errormsg as you have.
Also: something with your loadconfig must be wrong.
read the manual.
$ man ldconfig
check the config file for ldconfig. it is called /etc/ld.so.conf
Of course there should be no dirs mentioned twice, beware of symbolic links.
If the question is that you don't know what a symbolic link is:
read at the slackware site the 'book', or some other introduction in Linux.
Sometimes, if you build a program yourself after having download the tarball, it says you that you have to adapt the /etc/ld.so.conf file.
If you issue
# make install
a msg is issued on the screen...
This flies over the screen and you easily miss it.
#make install 2>&1 | tee reportfile
gives you all the stuff in a file too, which can be seen with less.
this too could be the source of your problems.
These are some thoughts, perhaps you have another problem....