pkg-config is a script to make putting together all the build
flags when compiling/linking a lot easier.

Report bugs at http://bugzilla.gnome.org (pkg-config is in no way 
gnome-specific but gnome.org was a convenient bug tracker).

To use pkg-config, do something like the following in your configure.in

    PKG_CHECK_MODULES([GNOME], [gtk > 1.2.8 gnomeui >= 1.2.0])

This puts the neccesary include flags to compile/link something against
libgnomeui and all its dependencies in $(GNOME_CFLAGS), and the -L/-l flags
for linking in $(GNOME_LIBS).

Users can define the PKG_CONFIG environment variable to point at the
right one, or if they cross-compile and have a correctly named pkg-config
(eg. arm-linux-pkg-config) in their PATH that will be used in preference.

Users can also define the GNOME_CFLAGS and GNOME_LIBS environment variables
if they think they know better, pkg-config will not be called if they do
that.

The "gtk > 1.2.8" part is only neccesary if you want to specifically check
if libgtk is version 1.2.8 or higher. Otherwise, the flags for gtk
will be included automatically, since libgnomeui depends on gtk.
So you could just say:

     PKG_CHECK_MODULES([GNOME], [gnomeui])

for any version of gnomeui.

For more info, there's even a man page, try 'man pkg-config'