Provide fallback if no arch flag is given

dune/codegen/sumfact/horizontaladd.hh

 #ifndef DUNE_CODEGEN_SUMFACT_HORIZONTALADD_HH
 #define DUNE_CODEGEN_SUMFACT_HORIZONTALADD_HH
 
-#include<immintrin.h>
 #include<dune/codegen/common/vectorclass.hh>
 
 
-template<class V>
-typename base_floatingpoint<V>::value horizontal_add_lower(const V& x)
-{
-  return horizontal_add(x.get_low());
-}
-
-template<class V>
-typename base_floatingpoint<V>::value horizontal_add_upper(const V& x)
-{
-  return horizontal_add(x.get_high());
-}
+// Only use our custom implementations if we have AVX2 or later!
+#if INSTRSET >= 8
 
 /** Implement a variant of horizontal_add(Vec2d) that avoids the haddpd
  *  instruction and instead uses the shuffle port.
@@ -49,6 +39,27 @@ static inline double permuting_horizontal_add(const Vec8d& a)
 
 #endif
 
+#else
+template<typename V>
+static inline double permuting_horizontal_add (const V& a)
+{
+    return horizontal_add(a);
+}
+
+#endif
+
+template<class V>
+typename base_floatingpoint<V>::value horizontal_add_lower(const V& x)
+{
+  return horizontal_add(x.get_low());
+}
+
+template<class V>
+typename base_floatingpoint<V>::value horizontal_add_upper(const V& x)
+{
+  return horizontal_add(x.get_high());
+}
+
 template<class V>
 typename base_floatingpoint<V>::value permuting_horizontal_add_lower(const V& x)
 {