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/build_detect_platform

@@ -129,11 +129,9 @@ case "$TARGET_OS" in
         ;;
     OS_WINDOWS_CROSSCOMPILE | NATIVE_WINDOWS)
         PLATFORM=OS_WINDOWS
-        COMMON_FLAGS="-fno-builtin-memcmp -D_REENTRANT -DOS_WINDOWS -DLEVELDB_PLATFORM_WINDOWS"
-        PLATFORM_SHARED_CFLAGS=""
+        COMMON_FLAGS="-fno-builtin-memcmp -D_REENTRANT -DOS_WINDOWS -DLEVELDB_PLATFORM_WINDOWS -DWINVER=0x0500 -D__USE_MINGW_ANSI_STDIO=1"
         PLATFORM_SOURCES="util/env_win.cc"
-        PLATFORM_CXXFLAGS="-std=c++0x"
-        PLATFORM_LIBS="-lshlwapi -ldbghelp"
+        PLATFORM_LIBS="-lshlwapi"
         PORT_FILE=port/port_win.cc
         CROSS_COMPILE=true
         ;;