diff --git a/configure.ac b/configure.ac
index 44d35e1..36e3fc5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -427,6 +427,10 @@ else
   fi
 fi
 
+if [[[ ! $CFLAGS[@] =~ "-O" ]]]; then
+  CFLAGS="$CFLAGS -O2"
+fi
+
 if test "$ac_env_CXXFLAGS_set" = set; then
   CXXFLAGS=$ac_env_CXXFLAGS_value
 elif test "${CXXFLAGS+set}" = set; then