LevelDB build bugfixes

* Bugfix: LevelDB: Use "xCXXFLAGS" variable to avoid user-defined CXXFLAGS from clobbering necessary compiler options
*	Bugfix: makefile.*: Move libleveldb.a dependency to OBJS so it doesn't clobber leveldb.o's dependency on leveldb.cpp
* Bugfix: makefile.{unix,osx}: clean target should clean LevelDB too