clang compiler warnings

[Werner LEMBERG]

Looking at some MacPorts log files from Mac OS 14.2.1 (which uses the
system's `clang` compiler) I see

```
flower/include/flower-proto.hh:26:30:
  warning: implicit conversion changes signedness:
           'int' to 'const vsize' (aka 'const unsigned long')
           [-Wsign-conversion]
inline constexpr vsize VPOS (-1);
                       ~~~~  ^~
```

for almost all C++ fies, making the log file quite noisy.  Dan, any
idea how to fix this gracefully?


    Werner