cdef classes: Forward declare with visibility flags (bug #59820). * libinterp/octave-value/cdef-fwd.h: Add header with forward declarations of cdef classes. * libinterp/octave-value/module.mk: Add new file to build system. * libinterp/octave-value/cdef-class.h, libinterp/octave-value/cdef-manager.h, libinterp/octave-value/cdef-method.h, libinterp/octave-value/cdef-object.h, libinterp/octave-value/cdef-package.h, libinterp/octave-value/cdef-property.h, libinterp/octave-value/cdef-utils.h: Include forward declaration header where needed. Export classes from library.

--- a/libinterp/octave-value/cdef-method.h	Thu Dec 16 22:01:14 2021 +0100
+++ b/libinterp/octave-value/cdef-method.h	Fri Dec 17 19:13:04 2021 +0100
@@ -40,6 +40,7 @@
 namespace octave
 {
   class
+  OCTINTERP_API
   cdef_method : public cdef_meta_object
   {
     friend class cdef_class;