Add an option to prune unwanted targets

test/prune_targets/program.cc

Index: test/prune_targets/program.cc
diff --git a/test/win/shard/hello2.cc b/test/prune_targets/program.cc
similarity index 93%
copy from test/win/shard/hello2.cc
copy to test/prune_targets/program.cc
index 23fcb546cb04a424be1c75f5c20d5950f0501095..529bf9b3a73c4727e06d85e249d800ba54ee1273 100644
--- a/test/win/shard/hello2.cc
+++ b/test/prune_targets/program.cc
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-int f2() {
+int main()
+{
   return 0;
 }