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

* Avoid writing build_config.h

* Remove build_config.h all together.

* Lint.

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