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

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