Add -flto to CXXFLAGS

Per openbmc/openbmc#3364, adding -flto to CXX flags
in order to reduce overall library and binary sizes.

Ref: https://gcc.gnu.org/wiki/LinkTimeOptimization

Change-Id: I3ee31a15fc0ded4308aa9d00a11b41bc62eb9799
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/presence/Makefile.am b/presence/Makefile.am
index b51563d..eb4dd6c 100644
--- a/presence/Makefile.am
+++ b/presence/Makefile.am
@@ -23,7 +23,8 @@
 	$(SDBUSPLUS_CFLAGS) \
 	$(PHOSPHOR_LOGGING_CFLAGS) \
 	${PHOSPHOR_DBUS_INTERFACES_CFLAGS} \
-	$(LIBEVDEV_CFLAGS)
+	$(LIBEVDEV_CFLAGS) \
+	-flto
 
 BUILT_SOURCES = generated.hpp
 TEMPLATES = \