The pkg-config program is used to retrieve information about installed
libraries in the system. It is typically used to compile and link against one
or more libraries. It's more useful to the compilation process than to
the end-user.

pkg-config retrieves information about packages from special metadata files.
These files are named after the package, with the extension .pc. By default,
pkg-config looks for these files in the following directories:
${PREFIX}/libdata/pkgconfig, ${LOCALBASE}/libdata/pkgconfig and
${X11BASE}/libdata/pkgconfig; it will also look in the list of directories
specified by the PKG_CONFIG_PATH environment variable.

WWW: http://www.freedesktop.org/wiki/Software_2fpkgconfig