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

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