[guile] Stop using deprecated aliases

The replacements should all work with Guile 1.8.0, which is the
oldest version we still aim to support.

Fixes #1624

Examples/test-suite/guile/guile_ext_test_external.cxx

@@ -19,6 +19,6 @@ SCM test_create()
 SCM test_is_pointer(SCM val)
 {
 #define FUNC_NAME "test-is-pointer"
-  return SCM_BOOL(SWIG_IsPointer(val));
+  return scm_from_bool(SWIG_IsPointer(val));
 #undef FUNC_NAME
 }