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.

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 in the following directories: ${PREFIX}/libdata/pkgconfig,
${LOCALBASE}/libdata/pkgconfig and ${X11BASE}/libdata/pkgconfig for these
files; it will also look in the list of directories specified by the
PKG_CONFIG_PATH environment variable.

The package name specified on the pkg-config command line is defined to be the
name of the metadata file, minus the .pc extension. If a library can install
multiple versions simultaneously, it must give each version its own name (for
example, GTK 1.2 might have the package name "gtk+" while GTK 2.0 has
"gtk+-2.0").

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