This removes a check for $ac_cv_prog_cc_c89 which is set by AC_PROG_CC
if defined(__STDC__) in the preprocessor. (Standard compliant compilers
are supposed to define __STDC__ to 1 but the value is actually not
checked here.)

Unfortunately, MSVC doesn't define it, so configure fails for MSVC.

This check is not very useful in practice. Over 30 years after C89 has
been released, there are no C compilers out there that are not
sufficiently compliant with C89 for the project. The only practically
relevant case was that the check rejected C++ compilers. A different
method to reject C++ compilers will be introduced in a later commit.

This adds MSVC builds built on Linux using wine. This requires some
settings of tools and flags because the autotools support for MSVC is
naturally somewhat limited.

The advantage of this approach is that it is compatible with our
existing CI scripts, so there's no need to write a Windows CI script
(in PowerShell or similar). If we want to test building and running on
Windows native (e.g., as supported by Cirrus CI) we could still do this
in the future.

Another advantage of this approach is that contributors can simply use
the docker image if they need a MSVC installation in a non-Windows
environment.

This commit also improves the Dockerfile by grouping RUN commands
according to Docker docs:
https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#run

This commit also raises the TEST_ITERS for wine tasks to the default.
The overhead of wine is negligible, so we can certainly afford the same
number of iterations as for native Linux tests.