make nullptr play nicely with g++4.6

* update configure check
* move nullptr_t to namespace Dune
* import nullptr_t from namespace std (according to updated proposal)

[[Imported from SVN: r6492]]

dune/common/nullptr.hh

@@ -28,7 +28,9 @@ private:
   void operator&() const;    // whose address can't be taken
 } nullptr = {};              // and whose name is nullptr
 
-typedef dune_nullptr_t nullptr_t;
+namespace Dune {
+  typedef dune_nullptr_t nullptr_t;
+}
 
 template<class T>
 bool operator == (T* t, dune_nullptr_t)
@@ -42,6 +44,14 @@ bool operator == (dune_nullptr_t, T* t)
   return (t == static_cast<T*>(nullptr));
 }
 
+#else
+
+#include <cstddef>
+
+namespace Dune {
+  using std::nullptr_t;
+}
+
 #endif // HAVE_NULLPTR
 
 #endif // DUNE_NULLPTR_HH

dune/common/test/nullptr-test.cc

@@ -8,7 +8,7 @@
 
 void basic_tests()
 {
-  typedef nullptr_t NULLPTR_T;
+  typedef Dune::nullptr_t NULLPTR_T;
   char* ch = nullptr;    // ch has the null pointer value
   char* ch2 = 0;         // ch2 has the null pointer value
 #ifdef FAIL

m4/cxx0x_nullptr.m4

@@ -3,7 +3,7 @@ AC_DEFUN([NULLPTR_CHECK],[
   AC_REQUIRE([GXX0X])
   AC_LANG_PUSH([C++])
   AC_MSG_CHECKING([whether nullptr is supported])
-  AC_TRY_COMPILE([],[typedef nullptr_t peng;
+  AC_TRY_COMPILE([],[
     char* ch = nullptr;
     ], [
     HAVE_NULLPTR=yes