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.cc

@@ -37,7 +37,7 @@ namespace leveldb {
 namespace port {
 
 Mutex::Mutex() :
-    cs_(nullptr) {
+    cs_(NULL) {
   assert(!cs_);
   cs_ = static_cast<void *>(new CRITICAL_SECTION());
   ::InitializeCriticalSection(static_cast<CRITICAL_SECTION *>(cs_));
@@ -48,7 +48,7 @@ Mutex::~Mutex() {
   assert(cs_);
   ::DeleteCriticalSection(static_cast<CRITICAL_SECTION *>(cs_));
   delete static_cast<CRITICAL_SECTION *>(cs_);
-  cs_ = nullptr;
+  cs_ = NULL;
   assert(!cs_);
 }
 
@@ -128,7 +128,7 @@ void InitOnce(OnceType* once, void (*initializer)()) {
 }
 
 void* AtomicPointer::Acquire_Load() const {
-  void * p = nullptr;
+  void * p = NULL;
   InterlockedExchangePointer(&p, rep_);
   return p;
 }