Make LTO controllable via travis env variable.

(cherry-picked from commit 66caa190f6)

utils/travis/steps/install.sh

@@ -8,15 +8,15 @@ export PLAY_TEST=true
 export MP_TEST=true
 export WML_TEST_TIME=15
 export BOOST_TEST=true
-export LTO=false
 
 if [ "$OPT" = "-O0" ]; then
     export EXTRA_FLAGS_RELEASE="-O0"
     export PLAY_TEST=false
     export MP_TEST=false
     export WML_TEST_TIME=20
-else
-    # change to true to enable LTO on optimized, non-xcode builds
+fi
+
+if [ "$LTO" == "" ]; then
     export LTO=false
 fi