Configure: Add backward compat fix for Guile 1.8.

Configure: Add backward compat fix for Guile 1.8.

Guile 1.9 introduces two new typedefs that are used with the
scm_internal_hash_fold() and scm_internal_hash_for_each_handle()
functions.

To preserve backward compatibility with Guile 1.8, we must check for
these typedefs in Guile's API, and conditionally compile them.

According to the Guile team, these two functions were intended for
internal use (thus their naming convention), so LilyPond should
eventually migrate away from using them.