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

to use it, do something like the following in your configure.in

    PKG_CHECK_MODULES(GNOME, gtk > 1.2.8 gnomeui >= 1.2.0)
    AC_SUBST(GNOME_CFLAGS)
    AC_SUBST(GNOME_LIBS)

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)

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'