gdk/gdk.h header is included and gdk_cairo_set_source_pixbuf symbol
is used from gdk, thus we need its header include path and link to it.
If clutter is built with gtk support, this is implicitly included via
clutter-1.0 dep, but it can also be built without gtk support, in
which case the build of libchamplain fails due to no gdk/gdk.h header
found.