The list of directories to be searched is stored in the file /etc/ld.so.conf. Simply append your desired directory to this file.
Now run ldconfig -v
Note: Don't do this for Gentoo. That already has /usr/local/lib included, and in any case /etc/ld.so.conf is automatically generated from the files in /etc/env.d, so any customizations you want must be made in the latter.
Note : ldconfig(8) must be run whenever a DLL is added, when a DLL is removed, or when the set of DLL directories changes
A colon-separated set of directories where libraries should be searched for first, before the standard set of directories.
Contains a list of external libraries to link with.
Note : The =<definition> part is optional.