Problem when cross-compiling with libc++ in mingw

Hello,
I am getting the following error from my compiler, in a relatively special build configuration:

```C++
/opt/mingw-llvm-toolchain/llvm-mingw/boost_1_67_0/boost/iostreams/positioning.hpp:52:10: error: no matching constructor for initialization of 'std::streampos' (aka 'fpos')
{ return std::streampos(std::mbstate_t(), off); }
^ ~~~~~~~~~~~~~~~~~~~~~
/opt/mingw-llvm-toolchain/llvm-mingw/x86_64-w64-mingw32/include/c++/v1/string:538:31: note: candidate constructor not viable: allows at most single argument '__off', but 2 arguments were provided
_LIBCPP_INLINE_VISIBILITY fpos(streamoff __off = streamoff()) : __st_(), __off_(__off) {}
^
/opt/mingw-llvm-toolchain/llvm-mingw/x86_64-w64-mingw32/include/c++/v1/string:532:28: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 2 were provided
class _LIBCPP_TEMPLATE_VIS fpos
^
/opt/mingw-llvm-toolchain/llvm-mingw/x86_64-w64-mingw32/include/c++/v1/string:532:28: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 2 were provided

...

/opt/mingw-llvm-toolchain/llvm-mingw/boost_1_67_0/boost/iostreams/positioning.hpp:96:16: error: no member named 'seekpos' in 'std::__1::fpos'
return pos.seekpos();
~~~ ^
```

My toolchain is as follows :

- Linux host
- clang / libc++ MinGW cross-compiler (e.G. clang is a linux binary which creates windows executables with the mingw API) : basically this : https://github.com/mstorsjo/llvm-mingw

Any idea of what's going on ? I suppose that `BOOST_IOSTREAMS_HAS_DINKUMWARE_FPOS` is defined when it shouldn't ?