PKG_CONFIG_PATH is an environment variable. From the man page for pkg-config:
A colon-separated (on Windows, semicolon-separated) list of
directories to search for .pc files. The default directory will
always be searched after searching the path; the default is lib-
dir/pkgconfig where libdir is the libdir where pkg-config was
You'll need to find the .pc file associated with the library and add that to the end of PKG_CONFIG_PATH just as you would if you were editing PATH. You can do it as a one-time thing on the command line like so:
Or you can edit /etc/profile to make the change permanent.