Looks like it's using GSubprocess even if configure says glib-2.35 should be enough:
x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I. -I..  -pthread -I/usr/include/gtk-3.0 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/gtk-3.0 -I/usr/include/gio-unix-2.0/ -I/usr/include/cairo -I/usr/include/pango-1.0 -I/usr/include/harfbuzz -I/usr/include/pango-1.0 -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libdrm -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libpng16 -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include  -pthread -I/usr/include/dbus-1.0 -I/usr/lib64/dbus-1.0/include -I/usr/include/json-glib-1.0 -I/usr/include/gnome-desktop-3.0 -I/usr/include/gtk-3.0 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/gtk-3.0 -I/usr/include/gio-unix-2.0/ -I/usr/include/cairo -I/usr/include/pango-1.0 -I/usr/include/harfbuzz -I/usr/include/pango-1.0 -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libdrm -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libpng16 -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/gsettings-desktop-schemas        -pthread -I/usr/include/gconf/2 -I/usr/include/dbus-1.0 -I/usr/lib64/dbus-1.0/include -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include   -pthread -I/usr/include/gio-unix-2.0/ -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include  -DLOCALE_DIR=\""/usr/share/locale"\" -DDATA_DIR=\""/usr/share/gnome-session"\" -DLIBEXECDIR=\"/usr/libexec\" -DGTKBUILDER_DIR=\""/usr/share/gnome-session"\" -DGCONF_SANITY_CHECK=\"""\" -DGCONFTOOL_CMD=\"\"  -Wall  -O2 -pipe -march=native -c -o gnome_session-gsm-shell-extensions.o `test -f 'gsm-shell-extensions.c' || echo './'`gsm-shell-extensions.c
gsm-manager.c: In function ‘on_display_server_failure’:
gsm-manager.c:316:18: warning: unused variable ‘allow_logout’ [-Wunused-variable]
gsm-process-helper.c:28:9: error: unknown type name ‘GSubprocess’
gsm-process-helper.c: In function ‘on_child_exited’:
gsm-process-helper.c:44:9: warning: implicit declaration of function ‘g_subprocess_wait_check_finish’ [-Wimplicit-function-declaration]
gsm-process-helper.c:44:47: error: ‘GSubprocess’ undeclared (first use in this function)
gsm-process-helper.c:44:47: note: each undeclared identifier is reported only once for each function it appears in
gsm-process-helper.c:44:59: error: expected expression before ‘)’ token
gsm-process-helper.c: In function ‘on_child_timeout’:
gsm-process-helper.c:60:9: warning: implicit declaration of function ‘g_subprocess_force_exit’ [-Wimplicit-function-declaration]
gsm-process-helper.c: In function ‘gsm_process_helper’:
gsm-process-helper.c:90:9: warning: implicit declaration of function ‘g_subprocess_newv’ [-Wimplicit-function-declaration]
gsm-process-helper.c:90:24: warning: assignment makes pointer from integer without a cast [enabled by default]
gsm-process-helper.c:94:9: warning: implicit declaration of function ‘g_subprocess_wait_async’ [-Wimplicit-function-declaration]
make[3]: *** [gnome_session-gsm-process-helper.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: Leaving directory `/var/tmp/portage/gnome-base/gnome-session-3.12.1/work/gnome-session-3.12.1/gnome-session'
make[2]: *** [all] Error 2
make[2]: Leaving directory `/var/tmp/portage/gnome-base/gnome-session-3.12.1/work/gnome-session-3.12.1/gnome-session'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/var/tmp/portage/gnome-base/gnome-session-3.12.1/work/gnome-session-3.12.1'
make: *** [all] Error 2