clear compiling warning

• remoev the define of shadow variables
  some local variables have same name

• remove the warning logical 'and' of equal expressions
  O_RDONLY == (O_RDONLY|O_LARGEFILE)

if (flags != O_RDONLY
#ifdef O_LARGEFILE
        && flags != (O_RDONLY|O_LARGEFILE)
#endif

• configure.ac: add option -Wno-nonnull-compare some function use the __nonnull ((1)) and do NULL compare