Update for changes to Whippet build

* libguile/Makefile.am (WHIPPET_EMBEDDER_H): Define this variable
instead of using -include; otherwise we don't get the chance to set
_LARGEFILE64_SOURCE before including Whippet files.
(AM_CPPFLAGS): Simplify.
(libguile_@GUILE_EFFECTIVE_VERSION@_la_DEPENDENCIES): Explicitly add
libwhippet.la as a dependency.
(noinst_HEADERS): Add gc-internal.h
* libguile/gc-internal.h: New file.
* libguile/gc.c: Include gc-internal.h.

libguile/gc-internal.h

@@ -0,0 +1,28 @@
+#ifndef SCM_GC_INTERNAL_H
+#define SCM_GC_INTERNAL_H
+
+/* Copyright 2025 Free Software Foundation, Inc.
+
+   This file is part of Guile.
+
+   Guile is free software: you can redistribute it and/or modify it
+   under the terms of the GNU Lesser General Public License as published
+   by the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   Guile is distributed in the hope that it will be useful, but WITHOUT
+   ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+   FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
+   License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with Guile.  If not, see
+   <https://www.gnu.org/licenses/>.  */
+
+
+
+#include <gc-api.h>
+
+
+
+#endif /* SCM_GC_INTERNAL_H */