--- a/configure.in
+++ b/configure.in
@@ -19,8 +19,8 @@ AC_CHECK_FUNCS(getopt)
 AC_CHECK_FUNCS(getopt_long)
 
 if test -n "$GCC"; then
-   CFLAGS="-O3  -Wno-deprecated -Wall"
-   CXXFLAGS="-O3 -Wno-deprecated -Wall"
+   CFLAGS+=" -Wno-deprecated -Wall"
+   CXXFLAGS+=" -Wno-deprecated -Wall"
 fi
 
 AC_ARG_WITH(