Recognize *-*-windows* config triplets

Recognize *-*-windows* config triplets

The gnuconfig project recognizes windows* as a host OS to denote native
Windows environments.  The commit message makes it sound like LLVM and
Crablang communities will use the 'windows' value, whereas GNU will
continue to use 'mingw'. But I think it's only a matter of time until
people start to pass the option --host=x86_64-pc-windows to configure
scripts. We should be prepared for that.

Url: https://savannah.gnu.org/patch/?10387

* build-aux/ltmain.in: Treat windows* as equivalent to mingw*.
* m4/libtool.m4: Likewise.
* m4/ltdl.m4: Likewise.
* m4/ltoptions.m4: Likewise.
* tests/bindir.at: Likewise.
* tests/deplibs-mingw.at: Likewise.
* tests/lt_dladvise.at: Likewise.
* tests/testsuite.at: Likewise.