The default settings for PKG_CONFIG_PATH are /lib/pkgconfig, /usr/lib/pkgconfig and /usr/local/lib/pkgconfig. These settings are hard-coded and do not have to be exported with the additional paths. You may add to PKG_CONFIG_PATH by exporting additional paths on your system where pkgconfig files are installed. Note that PKG_CONFIG_PATH is only needed when compiling packages, not during run-time.