Use cross-CPP for cross-build

Quoting Helmut Grohne on https://bugs.debian.org/1098407

guile-gnutls fails to cross build from source, because it uses the build architecture cpp via guile-snarf. While configure correctly detects the host one, it is not communicated to guile-snarf and thus it ends up using the wrong one. I'm attaching a patch for your convenience.

I have no idea about the CI errors (except for the copyright-year warning) I cannot see how they could be related to this change and do not see a testsuite error locally.