configure: Improve Xen autodetection for hosts without Xen

With this patch, it only takes one test (instead of four)
to detect that there is no Xen support at all.

For most build hosts, this will reduce the time configure needs.
It will also reduce noisy output in config.log.

Build hosts with Xen now need up to five (instead of up to four)
tests. They get improved diagnostics when Xen support fails.

Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1317,10 +1317,30 @@ fi
 if test "$xen" != "no" ; then
   xen_libs="-lxenstore -lxenctrl -lxenguest"
 
-  # Xen unstable
+  # First we test whether Xen headers and libraries are available.
+  # If no, we are done and there is no Xen support.
+  # If yes, more tests are run to detect the Xen version.
+
+  # Xen (any)
   cat > $TMPC <<EOF
 #include <xenctrl.h>
 #include <xs.h>
+int main(void) {
+  return 0;
+}
+EOF
+  if ! compile_prog "" "$xen_libs" ; then
+    # Xen not found
+    if test "$xen" = "yes" ; then
+      feature_not_found "xen"
+    fi
+    xen=no
+
+  # Xen unstable
+  elif (
+      cat > $TMPC <<EOF
+#include <xenctrl.h>
+#include <xs.h>
 #include <stdint.h>
 #include <xen/hvm/hvm_info_table.h>
 #if !defined(HVM_MAX_VCPUS)
@@ -1336,7 +1356,8 @@ int main(void) {
   return 0;
 }
 EOF
-  if compile_prog "" "$xen_libs" ; then
+      compile_prog "" "$xen_libs"
+    ) ; then
     xen_ctrl_version=410
     xen=yes
 
@@ -1407,10 +1428,10 @@ EOF
     xen_ctrl_version=330
     xen=yes
 
-  # Xen not found or unsupported
+  # Xen version unsupported
   else
     if test "$xen" = "yes" ; then
-      feature_not_found "xen"
+      feature_not_found "xen (unsupported version)"
     fi
     xen=no
   fi