Use `mmap()` because it works on Linux and OS X

The combination of `posix_memalign` and `mprotect` does not work as
expected on OS X, see also !43.

dune/common/debugallocator.hh

@@ -106,7 +106,7 @@ namespace Dune
             it->capacity << " bytes at " << it->ptr << std::endl;
             error = true;
           }
-          free(it->page_ptr);
+          munmap(it->page_ptr, it->pages * page_size);
         }
         if (error)
           allocation_error("lost allocations");
@@ -122,8 +122,10 @@ namespace Dune
         ai.pages = (ai.capacity) / page_size + 2;
         ai.not_free = true;
         size_type overlap = ai.capacity % page_size;
-        int result = posix_memalign(&(ai.page_ptr), page_size, ai.pages * page_size);
-        if (0 != result)
+        ai.page_ptr = mmap(NULL, ai.pages * page_size,
+                           PROT_READ | PROT_WRITE,
+                           MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+        if (MAP_FAILED == ai.page_ptr)
         {
           throw std::bad_alloc();
         }
@@ -171,7 +173,7 @@ namespace Dune
             memprotect(it->page_ptr,
                        (it->pages) * page_size,
                        PROT_READ | PROT_WRITE);
-            std::free(it->page_ptr);
+            munmap(it->page_ptr, it->pages * page_size);
             // remove chunk info
             allocation_list.erase(it);
 #endif