Enables link-time optimized builds for osrm binaries.

The default just flipped from on to off upstream.
Re-enables LTO. See Project-OSRM/osrm-backend#3524.

Makefile

@@ -19,7 +19,7 @@ node_modules: package.json
 build/%/node-osrm.node: ./node_modules
 	mkdir -p build &&\
 	 cd build &&\
-	 cmake .. -DCMAKE_BUILD_TYPE=$* -DBUILD_LIBOSRM=On &&\
+	 cmake .. -DCMAKE_BUILD_TYPE=$* -DBUILD_LIBOSRM=On -DENABLE_LTO=ON &&\
 	 VERBOSE=1 make -j${JOBS} &&\
 	 cd ..