Change detection of xen so that it's actually automatic rather than forced.

This ensures that ./configure will work fine if xen development packages
are not around, rather than fail. When passing ./configure --with-xen, the
lack of xen development packages become fatal.

configure.in

@@ -205,7 +205,7 @@ fi
 
 dnl Allow to build without Xen, QEMU/KVM, test or remote driver
 AC_ARG_WITH([xen],
-  AC_HELP_STRING([--with-xen], [add XEN support @<:@default=yes@:>@]),[],[with_xen=yes])
+  AC_HELP_STRING([--with-xen], [add XEN support @<:@default=check@:>@]),[],[with_xen=check])
 AC_ARG_WITH([xen-inotify],
   AC_HELP_STRING([--with-xen-inotify], [add XEN inotify support @<:@default=check@:>@]),[],[with_xen_inotify=check])
 AC_ARG_WITH([qemu],
@@ -334,6 +334,8 @@ if test "$with_libvirtd" = "yes" ; then
 fi
 AM_CONDITIONAL([WITH_LIBVIRTD], [test "$with_libvirtd" = "yes"])
 
+old_LIBS="$LIBS"
+old_CFLAGS="$CFLAGS"
 XEN_LIBS=""
 XEN_CFLAGS=""
 dnl search for the Xen store library
@@ -343,52 +345,58 @@ if test "$with_xen" != "no" ; then
         XEN_LIBS="-L$with_xen/lib64 -L$with_xen/lib"
     fi
     fail=0
-    old_LIBS="$LIBS"
-    old_CFLAGS="$CFLAGS"
     CFLAGS="$CFLAGS $XEN_CFLAGS"
     LIBS="$LIBS $XEN_LIBS"
     AC_CHECK_LIB([xenstore], [xs_read], [
            with_xen=yes
            XEN_LIBS="$XEN_LIBS -lxenstore"
        ],[
-           if test "$with_xen" = "check" ; then
-               with_xen=no
-           else
-               with_xen=no
+           if test "$with_xen" = "yes"; then
                fail=1
            fi
+           with_xen=no
        ])
+fi
 
-    test $fail = 1 &&
-      AC_MSG_ERROR([You must install the Xen development package to compile Xen driver with -lxenstore])
-
+if test "$with_xen" != "no" ; then
     AC_CHECK_HEADERS([xen/xen.h xen/version.h xen/dom0_ops.h],,[
-	AC_MSG_ERROR([Cannot find standard Xen headers. Is xen-devel installed?])
+       if test "$with_xen" = "yes"; then
+           fail=1
+       fi
+       with_xen=no
     ],
 [#include <stdio.h>
 #include <stdint.h>
 ])
+fi
 
+if test "$with_xen" != "no" ; then
     dnl Search for the location of <xen/{linux,sys}/privcmd.h>.
-    AC_CHECK_HEADERS([xen/sys/privcmd.h],,[
-	AC_CHECK_HEADERS([xen/linux/privcmd.h],,[
-	    AC_MSG_ERROR([Cannot find header file <xen/linux/privcmd.h> or <xen/sys/privcmd.h>.  Is xen-devel installed?])
-	],
-[#include <stdio.h>
-#include <stdint.h>
-#include <xen/xen.h>
-])
-    ],
-[#include <stdio.h>
-#include <stdint.h>
-#include <xen/xen.h>
-])
-    LIBS="$old_LIBS"
-    CFLAGS="$old_CFLAGS"
+    found=
+    AC_CHECK_HEADERS([xen/sys/privcmd.h xen/linux/privcmd.h], [found=yes; break;], [],
+       [#include <stdio.h>
+        #include <stdint.h>
+        #include <xen/xen.h>
+       ])
+    if test "x$found" != "xyes"; then
+        if test "$with_xen" = "yes"; then
+            fail=1
+        fi
+        with_xen=no
+    fi
 fi
+
+LIBS="$old_LIBS"
+CFLAGS="$old_CFLAGS"
+
+if test $fail = 1; then
+    AC_MSG_ERROR([You must install the Xen development package to compile Xen driver with -lxenstore])
+fi
+
 if test "$with_xen" = "yes"; then
     AC_DEFINE_UNQUOTED([WITH_XEN], 1, [whether Xen driver is enabled])
 fi
+
 AM_CONDITIONAL([WITH_XEN], [test "$with_xen" = "yes"])
 AC_SUBST([XEN_CFLAGS])
 AC_SUBST([XEN_LIBS])