meson.build: Skip C++ detection unless we're targeting Windows

The only C++ code that we currently still have in the repository
is the code in qga/vss-win32/ - so we can skip the C++ detection
unless we are compiling binaries for Windows.

Message-Id: <20230705133639.146073-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>