Building libctf on MinGW

[Nick Alcock]

On 17 Dec 2019, Eli Zaretskii stated:

> Hi,
>
> Back in November I built Binutils 2.33 with mingw.org's MinGW, and
> bumped into 3 compilation problems, which I reported on Bugzilla:
>
>   https://sourceware.org/bugzilla/show_bug.cgi?id=25155
>
> There was no response to that report, and these days I've built a
> pretest of GDB 9.1 and bumped into the same problem.

I'm sorry, I didn't notice this report because the component wasn't
"libctf": mea culpa. Clearly I need to be searching subjects as well :)

These should definitely be fixed, but to prevent the recurrence of
things like this I'd like to know how people compile on mingw. I've been
using the Cygwin->mingw cross-compilers, which show none of these
problems, so clearly a *proper* mingw compilation uses something
different. What is it? :)