Port leveldb to MinGW32

Several changes to make the native windows leveldb code compile
with mingw32 and run on 32-bit Windows:
* Remove -std=c++0x dependency (modified code to use NULL instead of
  nullptr)
* Link with -lshlwapi
* Only #define snprintf/etc if compiling with Visual Studio
* Do not link against DbgHelp.lib (wrote a CreateDir instead of using
  DbgHelp's MakeSureDirectoryPathExists
* Define WINVER=0x0500 so MinGW32 can use the 64-bit-filesystem Windows
  api calls
* Define __USE_MINGW_ANSI_STDIO=1 to use MinGW's printf (which supports
  %ll)

I also cleaned up makefile.mingw, assuming that dependencies would be in
the standard /usr/local/{include,lib} by default but allowing overriding
with make DEPSDIR=... etc

src/leveldb/port/port_win.h

@@ -31,9 +31,11 @@
 #ifndef STORAGE_LEVELDB_PORT_PORT_WIN_H_
 #define STORAGE_LEVELDB_PORT_PORT_WIN_H_
 
+#ifdef _MSC_VER
 #define snprintf _snprintf
 #define close _close
 #define fread_unlocked _fread_nolock
+#endif
 
 #include <string>
 #include <stdint.h>
@@ -120,7 +122,7 @@ class AtomicPointer {
  private:
   void * rep_;
  public:
-  AtomicPointer() : rep_(nullptr) { }
+  AtomicPointer() : rep_(NULL) { }
   explicit AtomicPointer(void* v); 
   void* Acquire_Load() const;