glr.cc: don't leak glr.c/glr.cc scaffolding to the user

Until we have a decent reimplementation of glr.cc, we have to use
tricks to shoehorn C++ symbols to the C engine of glr.c.  Some of them
are done via #define.  Unfortunately in Bison 3.6 some of these we
done in the header file, which broke valid user code.

Reported by Egor Pugin.
https://lists.gnu.org/r/bug-bison/2020-06/msg00003.html

* data/skeletons/glr.cc: Stop playing tricks with b4_pre_epilogue.
(b4_glr_cc_setup, b4_glr_cc_cleanup): New.
Much cleaner way to instal glr.cc's scaffolding around glr.c.
* data/skeletons/glr.c: Adjust to use them.

TODO

@@ -1,4 +1,7 @@
 * Bison 3.7
+** glr.cc
+Get rid of global_tokens_and_yystype.
+
 ** Bistromathic
 - Hitting tab on a line with a syntax error is ugly
 
@@ -148,6 +151,9 @@ Les catégories d'avertissements incluent :
 Line -1 and -3 should mention CATEGORIE, not CATEGORY.
 
 * Bison 3.8
+** Rewrite glr.cc
+Get rid of scaffolding in glr.c.
+
 ** Unit rules / Injection rules (Akim Demaille)
 Maybe we could expand unit rules (or "injections", see
 https://homepages.cwi.nl/~daybuild/daily-books/syntax/2-sdf/sdf.html), i.e.,