Minor modification to the configure.ac

Enables silent rules (use make V=1 to override)
Prints a summary after configure is completed

lib/algorithms/approx/bigfloat.h

@@ -322,7 +322,7 @@ public:
 
   friend bigfloat abs_bf(const bigfloat& x){
     bigfloat a;
-    a.x=abs(x.x);
+    a.x=fabs(x.x);
     return a;
   }