Use configure_file() to configure version only (#4974)

* Avoid writing build_config.h

* Remove build_config.h all together.

* Lint.

tests/cpp/c_api/test_c_api.cc

@@ -1,5 +1,6 @@
-// Copyright by Contributors
+// Copyright (c) 2019 by Contributors
 #include <gtest/gtest.h>
+#include <xgboost/version_config.h>
 #include <xgboost/c_api.h>
 #include <xgboost/data.h>
 

tests/cpp/common/test_version.cc

@@ -6,6 +6,7 @@
 #include <dmlc/filesystem.h>
 #include <dmlc/io.h>
 
+#include <xgboost/version_config.h>
 #include <xgboost/json.h>
 #include <xgboost/base.h>
 
@@ -58,4 +59,4 @@ TEST(Version, Basic) {
   str = str.substr(ptr);
   ASSERT_EQ(str.size(), 0);
 }
-}  // namespace xgboost
+}  // namespace xgboost

tests/cpp/test_learner.cc

@@ -2,9 +2,10 @@
 #include <gtest/gtest.h>
 #include <vector>
 #include "helpers.h"
+#include <dmlc/filesystem.h>
 
-#include "xgboost/learner.h"
-#include "dmlc/filesystem.h"
+#include <xgboost/learner.h>
+#include <xgboost/version_config.h>
 
 namespace xgboost {