Issue 2758. ly_module_lookup caused deprecation warnings with Guile V2.06.

Issue 2758. ly_module_lookup caused deprecation warnings with Guile V2.06.
The V2.17.0 definition of ly_module_lookup in module-scheme.cc uses two
functions, scm_sym2var and scm_module_lookup_closure, which issue
deprecation warnings in Guile V2.06.  A call to the new Guile API function,
scm_module_variable, provides the functionality for both deprecated
routines, but has not been back-ported to Guile V1.8.

This patch adds a conditionally-compiled block when running with a Guile
version < V2.0, so that Guile V1 will not use the deprecated scm_ calls
in ly_module_lookup, and Guile V2 will use API call scm_module_variable.