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

trunk/include/core/SkData.h

Index: trunk/include/core/SkData.h
===================================================================
--- trunk/include/core/SkData.h	(revision 8197)
+++ trunk/include/core/SkData.h	(working copy)
@@ -89,6 +89,12 @@
     static SkData* NewFromMalloc(const void* data, size_t length);
 
     /**
+     *  Create a new dataref from a pointer allocated by mmap. The Data object
+     *  will handle calling munmap().
+     */
+    static SkData* NewFromMMap(const void* data, size_t length);
+    
+    /**
      *  Create a new dataref using a subset of the data in the specified
      *  src dataref.
      */