Make tutorial examples use dynamic linkage. (#343)

* Add shared lib target for GPU version.

* Dynamically link tutorial examples.

* Add shared lib to clean target in GPU Makefile.

* Add clean target to tutorial examples Makefile.

* Minor modifications to tutorial GPU examples.

tutorial/cpp/4-GPU.cpp

@@ -36,8 +36,6 @@ int main() {
         xq[d * i] += i / 1000.;
     }
 
-    int k = 4;
-
     faiss::gpu::StandardGpuResources res;
 
     // Using a flat index
@@ -48,6 +46,8 @@ int main() {
     index_flat.add(nb, xb);  // add vectors to the index
     printf("ntotal = %ld\n", index_flat.ntotal);
 
+    int k = 4;
+
     {       // search xq
         long *I = new long[k * nq];
         float *D = new float[k * nq];