Remove plugin, cuda related code in automake & autoconf files (#4789)

* Build plugin example with CMake.

* Remove plugin, cuda related code in automake & autoconf files.

* Fix typo in GPU doc.

tests/cpp/CMakeLists.txt

@@ -12,7 +12,8 @@ if (USE_CUDA)
   file(GLOB_RECURSE CUDA_TEST_SOURCES "*.cu")
   list(APPEND TEST_SOURCES ${CUDA_TEST_SOURCES})
 endif (USE_CUDA)
-add_executable(testxgboost ${TEST_SOURCES} ${XGBOOST_OBJ_SOURCES})
+add_executable(testxgboost ${TEST_SOURCES} ${XGBOOST_OBJ_SOURCES}
+  ${xgboost_SOURCE_DIR}/plugin/example/custom_obj.cc)
 
 if (USE_CUDA)
   # OpenMP is mandatory for CUDA

tests/cpp/plugin/test_example_objective.cc

@@ -0,0 +1,15 @@
+#include <gtest/gtest.h>
+#include <xgboost/objective.h>
+#include <string>
+#include "../helpers.h"
+
+namespace xgboost {
+
+TEST(Plugin, ExampleObjective) {
+  xgboost::GenericParameter tparam = CreateEmptyGenericParam(0, 0);
+  auto * obj = xgboost::ObjFunction::Create("mylogistic", &tparam);
+  ASSERT_EQ(obj->DefaultEvalMetric(), std::string{"error"});
+  delete obj;
+}
+
+}  // namespace xgboost

tests/travis/run_test.sh

@@ -5,8 +5,6 @@ make -f dmlc-core/scripts/packages.mk lz4
 
 if [ ${TRAVIS_OS_NAME} == "osx" ]; then
     echo 'USE_OPENMP=0' >> config.mk
-    echo 'TMPVAR := $(XGB_PLUGINS)' >> config.mk
-    echo 'XGB_PLUGINS = $(filter-out plugin/lz4/plugin.mk, $(TMPVAR))' >> config.mk
 else
     # use g++-4.8 for linux
     export CXX=g++-4.8