move SK_MMAP_SUPPORT into SkPreConfig, so we can know about its availability

trunk/include/core/SkMMapStream.h

-
-/*
- * Copyright 2008 The Android Open Source Project
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-
-#ifndef SkMMapStream_DEFINED
-#define SkMMapStream_DEFINED
-
-#include "SkStream.h"
-
-class SkMMAPStream : public SkMemoryStream {
-public:
-    SkMMAPStream(const char filename[]);
-    virtual ~SkMMAPStream();
-
-    virtual void setMemory(const void* data, size_t length, bool);
-private:
-    void*   fAddr;
-    size_t  fSize;
-
-    void closeMMap();
-
-    typedef SkMemoryStream INHERITED;
-};
-
-#endif