Build failure in mingw toolchain

[Biswapriyo Nath]

I have been trying to compile guile with mingw toolchain for native
Win32 target. But the compilation process failed at the following
error:

```
ld.exe: .libs/libguile_3.0_la-posix.o: in function `scm_gethostname':
libguile/posix.c:2379: undefined reference to
`__imp_gethostname_used_without_requesting_gnulib_module_gethostname'
```

I found a similar thread[1] in gnulib but I don't know how to solve
the issue. How can I fix the build error?

If you need further information I can try to provide them. Thank you.

[1]: https://lists.gnu.org/archive/html/bug-gnulib/2010-09/msg00265.html