xenstat.plugin: check xenstat_vbd_error presence (netdata#7103)

The xenstat_vbd_error function is going to be part of the xen 4.13
release. The workaround here is to check if the function is already
present in configuration phase and set vbd_m->error always to 0 if
xenstat_vbd_error does not exist.

Author: arkamar

collectors/xenstat.plugin/xenstat_plugin.c

@@ -333,7 +333,11 @@ static int vbd_metrics_collect(struct domain_metrics *d, xenstat_domain *domain)
             return 1;
         }
 
+#ifdef HAVE_XENSTAT_VBD_ERROR
         vbd_m->error    = xenstat_vbd_error(vbd);
+#else
+        vbd_m->error    = 0;
+#endif
         vbd_m->oo_reqs  = xenstat_vbd_oo_reqs(vbd);
         vbd_m->rd_reqs  = xenstat_vbd_rd_reqs(vbd);
         vbd_m->wr_reqs  = xenstat_vbd_wr_reqs(vbd);

configure.ac

@@ -748,6 +748,22 @@ fi
 AC_MSG_RESULT([${enable_plugin_xenstat}])
 AM_CONDITIONAL([ENABLE_PLUGIN_XENSTAT], [test "${enable_plugin_xenstat}" = "yes"])
 
+if test "${enable_plugin_xenstat}" == "yes"; then
+    AC_MSG_CHECKING([for xenstat_vbd_error in -lxenstat])
+    AC_TRY_LINK(
+        [ #include <xenstat.h> ],
+        [
+            xenstat_vbd * vbd;
+            int out = xenstat_vbd_error(vbd);
+        ],
+        [
+            have_xenstat_vbd_error=yes
+            AC_DEFINE([HAVE_XENSTAT_VBD_ERROR], [1], [xenstat_vbd_error usability])
+        ],
+        [ have_xenstat_vbd_error=no ]
+    )
+    AC_MSG_RESULT([${have_xenstat_vbd_error}])
+fi
 
 # -----------------------------------------------------------------------------
 # perf.plugin