gnu: Add xgboost.

* gnu/packages/machine-learning.scm (xgboost): New variable.
* gnu/packages/patches/xgboost-use-system-dmlc-core.patch: New patch.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Leo Famulari <leo@famulari.name>

gnu/packages/patches/xgboost-use-system-dmlc-core.patch

@@ -0,0 +1,34 @@
+From 34167cc105e47589b86b6f62da76e0ab744002dd Mon Sep 17 00:00:00 2001
+From: Vinicius Monego <monego@posteo.net>
+Date: Tue, 11 May 2021 19:35:30 -0300
+Subject: [PATCH] Use dmlc-core from Guix.
+
+---
+This patch is a subset of the following Debian patch: https://sources.debian.org/src/xgboost/1.2.1-1/debian/patches/cmake.patch/
+
+Rabit is now developed as part of xgboost, so we don't build it separately.
+
+ CMakeLists.txt | 8 +++-----
+ 1 file changed, 3 insertions(+), 5 deletions(-)
+
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index 6cc19fbd..3b42bc17 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -150,11 +150,9 @@ endif (USE_OPENMP)
+
+ # dmlc-core
+ msvc_use_static_runtime()
+-add_subdirectory(${xgboost_SOURCE_DIR}/dmlc-core)
+-set_target_properties(dmlc PROPERTIES
+-  CXX_STANDARD 14
+-  CXX_STANDARD_REQUIRED ON
+-  POSITION_INDEPENDENT_CODE ON)
++add_library(dmlc SHARED IMPORTED)
++find_library(DMLC_LIBRARY dmlc)
++set_property(TARGET dmlc PROPERTY IMPORTED_LOCATION "${DMLC_LIBRARY}")
+ if (MSVC)
+   target_compile_options(dmlc PRIVATE
+                          -D_CRT_SECURE_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE)
+--
+2.31.1