cmake build system (#1314)

* Changed c api to compile under MSVC

* Include functional.h header for MSVC

* Add cmake build

include/xgboost/c_api.h

@@ -22,6 +22,7 @@ XGB_EXTERN_C {
 #else
 #define XGB_DLL XGB_EXTERN_C
 #endif
+
 // manually define unsign long
 typedef unsigned long bst_ulong;  // NOLINT(*)
 

include/xgboost/tree_updater.h

@@ -16,6 +16,10 @@
 #include "./data.h"
 #include "./tree_model.h"
 
+#ifdef _MSC_VER
+#include <functional>
+#endif
+
 namespace xgboost {
 /*!
  * \brief interface of tree update module, that performs update of a tree.