Make Clang compile cpp11_test.cpp without warnings

Warnings about unused variables are quenched by simply incrementing the
variables in question. This makes the try_compile() call in CMake
succeed without warnings when checking for C++11 compatibility and using
-Wall.

dlib/cmake_utils/test_for_cpp11/cpp11_test.cpp

@@ -26,8 +26,10 @@ public:
     {
         thread_local int stuff1 = 999;
         auto x = 4;
+        ++stuff1;
 
         decltype(x) asdf = 9;
+        ++asdf;
 
         auto f = []() { cout << "in a lambda!" << endl; };
         f();