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