configure: Clarify URL to source downloads

configure: Clarify URL to source downloads

If configure detects that it's being run on a source tree which
is missing git modules, it prints an error messages suggesting
that the user downloads a correct source archive from the project
website. However https://www.qemu.org/download/ is a link to a
page with multiple tabs, with the default being the one telling
users how to get binaries from their distro. Clarify the URL
we print to include the #source anchor, so that the browser will
go directly to the source-tarball instructions.

Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20190718131659.20783-1-peter.maydell@linaro.org
Suggested-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>