[Breaking] Add global versioning. (#4936)

* Use CMake config file for representing version.

* Generate c and Python version file with CMake.

The generated file is written into source tree.  But unless XGBoost upgrades
its version, there will be no actual modification.  This retains compatibility
with Makefiles for R.

* Add XGBoost version the DMatrix binaries.
* Simplify prefetch detection in CMakeLists.txt

include/xgboost/c_api.h

@@ -58,6 +58,16 @@ typedef struct {  // NOLINT(*)
   float* value;
 } XGBoostBatchCSR;
 
+/*!
+ * \brief Return the version of the XGBoost library being currently used.
+ *
+ *  The output variable is only written if it's not NULL.
+ *
+ * \param major Store the major version number
+ * \param minor Store the minor version number
+ * \param patch Store the patch (revision) number
+ */
+XGB_DLL void XGBoostVersion(int* major, int* minor, int* patch);
 
 /*!
  * \brief Callback to set the data to handle,