With lightdm 1.1.1 (where gio-2.0 was added but everything was moved to Requires.private) I still get (with make V=1)


make[2]: Entering directory `/tmp/buildd/lightdm-gtk-greeter-1.1.1/src'
/bin/bash ../libtool  --tag=CC   --mode=link gcc -DGETTEXT_PACKAGE=\"lightdm-gtk-greeter\" -DLOCALEDIR=\""/usr/share/locale"\" -DGREETER_DATA_DIR=\"""\" -DCONFIG_FILE=\""/lightdm-gtk-greeter.conf"\" -DGSEAL_ENABLE -pthread -I/usr/include/gtk-3.0 -I/usr/include/atk-1.0 -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/pango-1.0 -I/usr/include/gio-unix-2.0/ -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng12 -I/usr/include/lightdm-gobject-1 -I/usr/include/cairo -I/usr/include/libxml2   -Wall -Wmissing-prototypes -Wnested-externs -Wpointer-arith -Wno-sign-compare  -g -O2 -fPIE -fstack-protector --param=ssp-buffer-size=4 -Wformat -Wformat-security -Werror=format-security  -fPIE -pie -Wl,-z,relro -Wl,-z,now -Wl,--as-needed -Wl,-O1 -o lightdm-gtk-greeter lightdm_gtk_greeter-lightdm-gtk-greeter.o -pthread -Wl,--export-dynamic -lgtk-3 -lgdk-3 -latk-1.0 -lgio-2.0 -lpangoft2-1.0 -lpangocairo-1.0 -lgdk_pixbuf-2.0 -lcairo-gobject -lcairo -lpango-1.0 -lfreetype -lfontconfig -lgobject-2.0 -lgthread-2.0 -llightdm-gobject-1 -lgmodule-2.0 -lrt -lglib-2.0   
libtool: link: gcc -DGETTEXT_PACKAGE=\"lightdm-gtk-greeter\" -DLOCALEDIR=\"/usr/share/locale\" -DGREETER_DATA_DIR=\"\" -DCONFIG_FILE=\"/lightdm-gtk-greeter.conf\" -DGSEAL_ENABLE -pthread -I/usr/include/gtk-3.0 -I/usr/include/atk-1.0 -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/pango-1.0 -I/usr/include/gio-unix-2.0/ -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng12 -I/usr/include/lightdm-gobject-1 -I/usr/include/cairo -I/usr/include/libxml2 -Wall -Wmissing-prototypes -Wnested-externs -Wpointer-arith -Wno-sign-compare -g -O2 -fPIE -fstack-protector --param=ssp-buffer-size=4 -Wformat -Wformat-security -Werror=format-security -fPIE -pie -Wl,-z -Wl,relro -Wl,-z -Wl,now -Wl,--as-needed -Wl,-O1 -o lightdm-gtk-greeter lightdm_gtk_greeter-lightdm-gtk-greeter.o -pthread -Wl,--export-dynamic  -lgtk-3 -lgdk-3 -latk-1.0 -lgio-2.0 -lpangoft2-1.0 -lpangocairo-1.0 -lgdk_pixbuf-2.0 /usr/lib/libcairo-gobject.so /usr/lib/libcairo.so -lpango-1.0 /usr/lib/x86_64-linux-gnu/libfreetype.so -lfontconfig -lgobject-2.0 -lgthread-2.0 -llightdm-gobject-1 -lgmodule-2.0 -lrt -lglib-2.0 -pthread
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/liblightdm-gobject-1.a(liblightdm_gobject_1_la-user.o): undefined reference to symbol 'g_file_monitor'
/usr/bin/ld: note: 'g_file_monitor' is defined in DSO /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/libgio-2.0.so so try adding it to the linker command line
/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/libgio-2.0.so: could not read symbols: Invalid operation
collect2: ld returned 1 exit status
make[2]: *** [lightdm-gtk-greeter] Error 1
make[2]: Leaving directory `/tmp/buildd/lightdm-gtk-greeter-1.1.1/src'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/tmp/buildd/lightdm-gtk-greeter-1.1.1'
make: *** [all] Error 2