buildtools: Expose the Python 3 ABI tag

buildtools: Expose the Python 3 ABI tag

Expose the tag in the env to allow using it in pkg-config files

Add a "pyembed_libname" function to correctly form library names.
(This can't be done automatically in SAMBA_LIBRARY because the name
could be used as a dependency for another library)

Signed-off-by: Petr Viktorin <pviktori@redhat.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jelmer Vernooij <jelmer@samba.org>