suspend: imported suspend/resume support from NetworkManager

We're going to make this optional during build, not mandatory as was the case
in NM.

src/Makefile.am

@@ -240,6 +240,16 @@ ModemManager_LDADD += $(POLKIT_LIBS)
 ModemManager_CPPFLAGS += $(POLKIT_CFLAGS)
 endif
 
+# Additional suspend/resume support via systemd
+if SUSPEND_RESUME_SYSTEMD
+ModemManager_SOURCES += mm-sleep-monitor.h mm-sleep-monitor-systemd.c
+endif
+
+# Additional suspend/resume support via upower
+if SUSPEND_RESUME_UPOWER
+ModemManager_SOURCES += mm-sleep-monitor.h mm-sleep-monitor-upower.c
+endif
+
 # Additional QMI support in ModemManager
 if WITH_QMI
 ModemManager_SOURCES += \