Your package config files end in .pc
Search your systems for file that end in .pc and those are the ones you want.
Common locations include /usr/local/kde/share/config; /usr/local/kde/lib/pkgconfig; /usr/local/lib/pkgconfig; /usr/lib/pkgconfig;/usr/X11R6/lib/pkgconfig
There are, of course, other locations.
I have gotten into the habit of symlinking the most common .pc files into other locations in order to deal with the rather annoying problem that you are describing. This way I don't have to remember which files are in which locations, and I don't have to sort out nearly as often what path to put into the PKG_CONFIG_PATH.