build: Add -flto to CXXFLAGS

Add -flto to CXXFLAGS to allow build optimization.

Change-Id: Ide77369702db4e0ab687143649a2e359aeb83f94
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index 69d58a1..3d598ba 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -7,4 +7,4 @@
 libi2ccmds_la_LDFLAGS = \
 	-version-info 0:0:0 -shared
 
-libi2ccmds_la_CXXFLAGS =
+libi2ccmds_la_CXXFLAGS = -flto