build-sys: do no support modules on Windows

build-sys: do no support modules on Windows

Our module system does not support Windows, because it relies on
resolving symbols from the main executable.

If there is enough interest in supporting modules on Windows, we could
generate an import library for the executable and link with it:
https://stackoverflow.com/questions/15454968/dll-plugin-that-uses-functions-defined-in-the-main-executable

However, there is a small chicken egg problem, since the executable
link and exports extra symbols needed by the library...

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190718120413.27678-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>