gdl fails to build outside source tree (builddir != srcdir), with following error:

---8<---
Making all in gdl
make[2]: Entering directory `/home/thep/build/gnome_git/gdl/gdl'
glib-genmarshal \
                --prefix=gdl_marshal /home/thep/vcs/gnome_git/gdl/gdl/libgdlmarshal.list --header > xgen-gmc \
        && cp xgen-gmc libgdlmarshal.h \
        && rm -f xgen-gmc
echo "#include \"libgdlmarshal.h\"" > xgen-gmc \
        && glib-genmarshal \
                --prefix=gdl_marshal /home/thep/vcs/gnome_git/gdl/gdl/libgdlmarshal.list --body >> xgen-gmc \
        && cp xgen-gmc libgdlmarshal.c \
        && rm -f xgen-gmc
glib-mkenums \
                        --fhead "#include <glib-object.h>\n" \
                        --fhead "#include \"libgdltypebuiltins.h\"\n\n" \
                        --fprod "\n/* enumerations from \"@filename@\" */" \
                        --vhead "static const G@Type@Value _@enum_name@_values[] = {" \
                        --vprod "  { @VALUENAME@, \"@VALUENAME@\", \"@valuenick@\" }," \
                        --vtail "  { 0, NULL, NULL }\n};\n\n" \
                        --vtail "GType\n@enum_name@_get_type (void)\n{\n" \
                        --vtail "  static GType type = 0;\n\n" \
                        --vtail "  if (!type)\n" \
                        --vtail "    type = g_@type@_register_static (\"@EnumName@\", _@enum_name@_values);\n\n" \
                        --vtail "  return type;\n}\n\n" \
                gdl.h gdl-tools.h gdl-dock-object.h gdl-dock-master.h gdl-dock.h gdl-dock-item.h gdl-dock-item-grip.h gdl-dock-notebook.h gdl-dock-paned.h gdl-dock-tablabel.h gdl-dock-layout.h gdl-dock-placeholder.h gdl-dock-bar.h gdl-switcher.h > xgen-gtbec \
        && cp xgen-gtbec libgdltypebuiltins.c \
        && rm -f xgen-gtbec
Can't open gdl.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-tools.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-object.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-master.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-item.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-item-grip.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-notebook.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-paned.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-tablabel.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-layout.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-placeholder.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-bar.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-switcher.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
glib-mkenums \
                        --fhead "#ifndef __LIBGDLTYPEBUILTINS_H__\n" \
                        --fhead "#define __LIBGDLTYPEBUILTINS_H__ 1\n\n" \
                        --fhead "#include <gdl/gdl.h>\n\n" \
                        --fhead "G_BEGIN_DECLS\n\n" \
                        --ftail "G_END_DECLS\n\n" \
                        --ftail "#endif /* __LIBGDLTYPEBUILTINS_H__ */\n" \
                        --fprod "\n/* --- @filename@ --- */" \
                        --eprod "#define GDL_TYPE_@ENUMSHORT@ @enum_name@_get_type()\n" \
                        --eprod "GType @enum_name@_get_type (void);\n" \
                gdl.h gdl-tools.h gdl-dock-object.h gdl-dock-master.h gdl-dock.h gdl-dock-item.h gdl-dock-item-grip.h gdl-dock-notebook.h gdl-dock-paned.h gdl-dock-tablabel.h gdl-dock-layout.h gdl-dock-placeholder.h gdl-dock-bar.h gdl-switcher.h > xgen-gth \
        && cp xgen-gth libgdltypebuiltins.h \
        && rm -f xgen-gth
Can't open gdl.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-tools.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-object.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-master.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-item.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-item-grip.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-notebook.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-paned.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-tablabel.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-layout.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-placeholder.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-dock-bar.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
Can't open gdl-switcher.h: No such file or directory at /home/gnome2/bin/glib-mkenums line 259.
make  all-am
make[3]: Entering directory `/home/thep/build/gnome_git/gdl/gdl'
...
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I/home/thep/vcs/gnome_git/gdl/gdl -I.. -DG_LOG_DOMAIN=\"Gdl\" -DGDL_GLADEDIR=\"/home/gnome2/share/gdl/glade\" -I/home/gnome2/include -I/home/thep/vcs/gnome_git/gdl -DGDK_DISABLE_DEPRECATED -DGTK_DISABLE_DEPRECATED -DGNOME_DISABLE_DEPRECATED -DBONOBO_DISABLE_DEPRECATED -DBONOBO_UI_DISABLE_DEPRECATED -I/home/gnome2/include/gtk-2.0 -I/home/gnome2/lib64/gtk-2.0/include -I/home/gnome2/include/atk-1.0 -I/home/gnome2/include/cairo -I/home/gnome2/include/pango-1.0 -I/home/gnome2/include/glib-2.0 -I/home/gnome2/lib64/glib-2.0/include -I/home/gnome2/include/pixman-1 -I/home/gnome2/include/libglade-2.0 -I/home/gnome2/include/libxml2 -I/usr/include/freetype2 -I/usr/include/libpng12 -I/home/gnome2/include/libxml2 -g -O2 -I/home/gnome2/include -MT gdl-dock-object.lo -MD -MP -MF .deps/gdl-dock-object.Tpo -c /home/thep/vcs/gnome_git/gdl/gdl/gdl-dock-object.c  -fPIC -DPIC -o .libs/gdl-dock-object.o
/home/thep/vcs/gnome_git/gdl/gdl/gdl-dock-object.c: In function ‘gdl_dock_object_class_init’:
/home/thep/vcs/gnome_git/gdl/gdl/gdl-dock-object.c:184: error: ‘GDL_TYPE_DOCK_PLACEMENT’ undeclared (first use in this function)
/home/thep/vcs/gnome_git/gdl/gdl/gdl-dock-object.c:184: error: (Each undeclared identifier is reported only once
/home/thep/vcs/gnome_git/gdl/gdl/gdl-dock-object.c:184: error: for each function it appears in.)
/home/thep/vcs/gnome_git/gdl/gdl/gdl-dock-object.c: In function ‘gdl_dock_param_get_type’:
/home/thep/vcs/gnome_git/gdl/gdl/gdl-dock-object.c:855: error: ‘GDL_TYPE_DOCK_PLACEMENT’ undeclared (first use in this function)
make[3]: *** [gdl-dock-object.lo] Error 1
make[3]: Leaving directory `/home/thep/build/gnome_git/gdl/gdl'
make[2]: *** [all] Error 2
make[2]: Leaving directory `/home/thep/build/gnome_git/gdl/gdl'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/thep/build/gnome_git/gdl'
make: *** [all] Error 2
---8<---

When builddir != srcdir, the glib-mkenums rules just fail to access source files unless VPATH is employed in the rule.