Added a pragma statement that tells visual studio to not recursively inline

functions very much when using the dnn tools, since otherwise it will sometimes
take hours to compile.

dlib/dnn/core.h

@@ -20,6 +20,12 @@
 #include <type_traits>
 #include "../metaprogramming.h"
 
+#ifdef _MSC_VER
+// Tell Visual Studio not to recursively inline functions very much because otherwise it
+// takes hours to compile the DNN code sometimes.  It's crazy.  Hopefully we can remove
+// this some day when the visual studio compiler is more efficient.
+#pragma inline_depth(2)
+#endif
 
 
 namespace dlib