[FIX] fix plugin system

Makefile

@@ -1,8 +1,3 @@
-# flags by plugin
-PLUGIN_OBJS=
-PLUGIN_LDFLAGS=
-PLUGIN_CFLAGS=
-
 ifndef config
 ifneq ("$(wildcard ./config.mk)","")
 	config = config.mk
@@ -28,6 +23,9 @@ ifeq ($(USE_OPENMP), 0)
 endif
 include $(DMLC_CORE)/make/dmlc.mk
 
+# include the plugins
+include $(XGB_PLUGINS)
+
 # use customized config file
 ifndef CC
 export CC  = $(if $(shell which gcc-5),gcc-5,gcc)