r103: Check for the right symbols in nbd.h by compiling, rather than guessing on

whether they'll be there on the basis of other, unrelated, symbols. Also, make
the error message that appears if we don't find what we need a bit more
helpful, and warn the user if a bad nbd.h is found locally but a good one is
provided system-wide.

diff --git a/configure.ac b/configure.ac
index 5965ad3..80f6181 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -74,22 +74,31 @@ dnl We need to check for NBD_CMD_DISC, but that's part of an enum, it is not
 dnl #define'd. Therefore, we check for something which is differently #define'd
 dnl in the old or new versions, even if we don't really care about that.
 dnl This might break at some time, but it should work for now, so...
-AC_TRY_CPP([#include "nbd.h"
-#ifdef LOCAL_END_REQUEST
-#error not here
-#endif
-], 
-  [AC_DEFINE(NBD_H_LOCAL, 1, Set to 1 if a (recent enough) nbd.h can be found in the current directory)
-    NBD_H='"nbd.h"'],
-  AC_TRY_CPP([#include <linux/nbd.h>
-#ifdef LOCAL_END_REQUEST
-#error not here
-#endif
+AC_TRY_COMPILE([#include <stdio.h>
+#define MY_NAME "conftest"
+#define NBD_H_LOCAL
+#include "cliserv.h"
 ],
-    [AC_DEFINE(NBD_H_LINUX, 1, Set to 1 if a (recent enough) nbd.h can be found in the linux directory in the search path) NBD_H='<linux/nbd.h>'],
-    AC_MSG_ERROR(Could not find a (recent enough) nbd.h)
+[int foo=NBD_CMD_DISC], 
+  [AC_DEFINE(NBD_H_LOCAL, 1, Set to 1 if a (2.6) nbd.h can be found in the current directory)
+    NBD_H='"nbd.h"'],
+  AC_TRY_COMPILE([#include <stdio.h>
+#define MY_NAME "conftest"
+#define NBD_H_LINUX
+#include "cliserv.h"
+  ],
+[int foo=NBD_CMD_DISC],
+    [AC_DEFINE(NBD_H_LINUX, 1, Set to 1 if a (2.6) nbd.h can be found in the linux directory in the search path) NBD_H='<linux/nbd.h>'],
+    AC_MSG_ERROR(Could not find an nbd.h from 2.6 or above.)
   )
 )
+if test -f nbd.h
+then
+  if test $NBD_H='<linux/nbd.h>'
+  then
+    AC_MSG_WARN([there is a local nbd.h, but it is from the 2.4-series of kernels. Using the system-provided, working, nbd.h])
+  fi
+fi
 AC_MSG_RESULT($NBD_H)
 AC_CHECK_HEADERS([sys/mount.h],,,
 [[#include <sys/param.h>