Replace pthread_exit(NULL) calls by return NULL

This MR solves a segfault problem with pthread. See the linked issue for details. It follows the accepted answer in https://stackoverflow.com/a/27235846

Closes #121 (closed)

This seems to solve the issues when tested with clang libc++ (where the error actually showed up)

dune/istl/paamg/test/pthreadamgtest.cc

@@ -94,7 +94,7 @@ void *solve(void* arg)
 
   std::cout<<"AMG solving took "<<solvetime<<" seconds"<<std::endl;
 
-  pthread_exit(NULL);
+  return NULL;
 }
 
 void *solve2(void* arg)