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
         ;;

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;
 }

src/leveldb/port/port_win.h

@@ -31,9 +31,11 @@
 #ifndef STORAGE_LEVELDB_PORT_PORT_WIN_H_
 #define STORAGE_LEVELDB_PORT_PORT_WIN_H_
 
+#ifdef _MSC_VER
 #define snprintf _snprintf
 #define close _close
 #define fread_unlocked _fread_nolock
+#endif
 
 #include <string>
 #include <stdint.h>
@@ -120,7 +122,7 @@ class AtomicPointer {
  private:
   void * rep_;
  public:
-  AtomicPointer() : rep_(nullptr) { }
+  AtomicPointer() : rep_(NULL) { }
   explicit AtomicPointer(void* v); 
   void* Acquire_Load() const;
 

src/leveldb/util/env_win.cc

@@ -20,9 +20,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include <io.h>
-#include <dbghelp.h>
 #include <algorithm>
-#pragma comment(lib,"DbgHelp.lib")
 
 #ifdef max
 #undef max
@@ -908,18 +906,34 @@ uint64_t Win32Env::NowMicros()
     return (uint64_t)(GetTickCount64()*1000);
 }
 
-Status Win32Env::CreateDir( const std::string& dirname )
+static Status CreateDirInner( const std::string& dirname )
 {
     Status sRet;
+    DWORD attr = ::GetFileAttributes(dirname.c_str());
+    if (attr == INVALID_FILE_ATTRIBUTES) { // doesn't exist:
+      std::size_t slash = dirname.find_last_of("\\");
+      if (slash != std::string::npos){
+	sRet = CreateDirInner(dirname.substr(0, slash));
+	if (!sRet.ok()) return sRet;
+      }
+      BOOL result = ::CreateDirectory(dirname.c_str(), NULL);
+      if (result == FALSE) {
+	sRet = Status::IOError(dirname, "Could not create directory.");
+	return sRet;
+      }
+    }
+    return sRet;
+}
+
+Status Win32Env::CreateDir( const std::string& dirname )
+{
     std::string path = dirname;
     if(path[path.length() - 1] != '\\'){
         path += '\\';
     }
     ModifyPath(path);
-    if(!::MakeSureDirectoryPathExists( path.c_str() ) ){
-        sRet = Status::IOError(dirname, "Could not create directory.");
-    }
-    return sRet;
+
+    return CreateDirInner(path);
 }
 
 Status Win32Env::DeleteDir( const std::string& dirname )