configure: Fix test for python 3.8

configure: Fix test for python 3.8

https://docs.python.org/3.8/whatsnew/3.8.html#debug-build-uses-the-same-abi-as-release-build

> To embed Python into an application, a new --embed option must be
> passed to python3-config --libs --embed to get -lpython3.8 (link the
> application to libpython). To support both 3.8 and older, try
> python3-config --libs --embed first and fallback to python3-config
> --libs (without --embed) if the previous command fails.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Wei Liu <wl@xen.org>
[ wei: rerun autogen.sh ]