glr.c: fix signature when using custom error messages

Reported by Tom Shields <[email protected]>.

* data/skeletons/glr.c (yypcontext_location): Fix return type.
* tests/calc.at: Check the case pure, location, custom error messages.

TODO

@@ -185,6 +185,8 @@ Line -1 and -3 should mention CATEGORIE, not CATEGORY.
 
 * Bison 3.8
 ** Rewrite glr.cc (currently glr2.cc)
+*** custom error messages
+
 *** Remove jumps
 We can probably replace setjmp/longjmp with exceptions.  That would help
 tremendously other languages such as D and Java that probably have no