With Boost 1.78 (installed on my system), I get a compiler error:

```
./fs.h: In function ‘fs::path fs::operator+(fs::path, fs::path)’:
./fs.h:91:8: error: no match for ‘operator+=’ (operand types are ‘fs::path’ and ‘std::remove_reference<fs::path&>::type’ {aka ‘fs::path’})
   91 |     p1 += std::move(p2);
      |     ~~~^~~~~~~~~~~~~~~~
```

It appears that in Boost 1.78, `operator+=` became a template function, and our `path` class which subclasses `boost::filesystem::path::path` no longer has an `operator+=` which will work with it.

I also opened an issue with boost: https://github.com/boostorg/filesystem/issues/223
{
public:
    using boost::filesystem::path::path;
    
    path(boost::filesystem::path path) : boost::filesystem::path::path(std::move(path)) {}
    path& operator=(boost::filesystem::path path) { boost::filesystem::path::operator=(std::move(path)); return *this; }
    path& operator/=(boost::filesystem::path path) { boost::filesystem::path::operator/=(std::move(path)); return *this; }
    
    path(const char* c) : boost::filesystem::path(c) {}