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

bitcoin-qt.pro

@@ -105,13 +105,14 @@ LIBS += $$PWD/src/leveldb/libleveldb.a $$PWD/src/leveldb/libmemenv.a
     isEmpty(QMAKE_RANLIB) {
         QMAKE_RANLIB = $$replace(QMAKE_STRIP, strip, ranlib)
     }
-    genleveldb.commands = cd $$PWD/src/leveldb && CC=$$QMAKE_CC CXX=$$QMAKE_CXX TARGET_OS=OS_WINDOWS_CROSSCOMPILE CXXFLAGS="-I$$BOOST_INCLUDE_PATH" LDFLAGS="-L$$BOOST_LIB_PATH" $(MAKE) libleveldb.a libmemenv.a && $$QMAKE_RANLIB $$PWD/src/leveldb/libleveldb.a && $$QMAKE_RANLIB $$PWD/src/leveldb/libmemenv.a
+    LIBS += -lshlwapi
+    genleveldb.commands = cd $$PWD/src/leveldb && CC=$$QMAKE_CC CXX=$$QMAKE_CXX TARGET_OS=OS_WINDOWS_CROSSCOMPILE $(MAKE) libleveldb.a libmemenv.a && $$QMAKE_RANLIB $$PWD/src/leveldb/libleveldb.a && $$QMAKE_RANLIB $$PWD/src/leveldb/libmemenv.a
 }
 genleveldb.target = $$PWD/src/leveldb/libleveldb.a
 genleveldb.depends = FORCE
 PRE_TARGETDEPS += $$PWD/src/leveldb/libleveldb.a
 QMAKE_EXTRA_TARGETS += genleveldb
-# Gross ugly hack that depends on qmake internals, unfortunately there's no other way to do it.
+# Gross ugly hack that depends on qmake internals, unfortunately there is no other way to do it.
 QMAKE_CLEAN += $$PWD/src/leveldb/libleveldb.a; cd $$PWD/src/leveldb ; $(MAKE) clean
 
 # regenerate src/build.h