bugfix: Check for __attribute__((deprecated)) only works when compiled as C++

[[Imported from SVN: r5212]]

m4/dune_deprecated.m4

@@ -4,6 +4,7 @@
 
 AC_DEFUN([DUNE_CHECKDEPRECATED],[
 	AC_MSG_CHECKING([for __attribute__((deprecated))])
+        AC_LANG_PUSH([C++])
         AC_TRY_COMPILE([#define DEP __attribute__((deprecated))
                     class bar { bar() DEP; };
                     class peng { } DEP;
@@ -18,6 +19,8 @@ AC_DEFUN([DUNE_CHECKDEPRECATED],[
 				   [DUNE_DEPRECATED=""
                     AC_MSG_RESULT(no)])
 
+        AC_LANG_POP([C++])
+
     AC_DEFINE_UNQUOTED(DUNE_DEPRECATED, $DUNE_DEPRECATED,
                       [how to create a deprecated warning])
 ])