libtool: Handle unset $multilib for Windows targets.

libtool: Handle unset $multilib for Windows targets.

If `multilib` is unset (which it apparently is by default), the output
from running a `configure` script contains lines like the following:
```
checking dynamic linker characteristics... ../configure: line 26791: test: yes: unary operator expected
Win32 ld.exe
```

The additional output is not a fatal error. But it could be irritating.

Avoid the unexpected output from `test` by making sure `=` is used as a
binary operator.

* m4/libtool.m4: Prepend "x" on both sides of a comparison with `test`
and the operator `=`.
* NEWS: Update for bug fix.

See also: https://github.com/msys2/MINGW-packages/pull/21905