m4: catch suspicions of unevaluated macros

Check in m4's output if there are sequences such as m4_foo or b4_foo,
which are probably resulting from incorrect m4 processing.

It actually already is useful:

- it caught a leaking b4_lac_if leaking from glr.c, where LAC is not
  supported, hence b4_lac_if is not defined.

- it also caught references to location.hh in position.hh when
  location.hh does not exist.

- while making "Code injection" robust to these new warnings (it is
  its very purpose to let b4_canary pass unevaluated), I saw that it
  did not check lalr1.d, and when adding lalr1.d, it revealed it did
  underquote ocurrences of token value types.

* src/scan-skel.l (macro): New abbreviation.
Use it.
* data/skeletons/glr.c: Don't use b4_lac_if, we don't have it.
* data/skeletons/location.cc: Don't generate position.hh when we don't
generate location.hh.
* data/skeletons/d.m4 (b4_basic_symbol_constructor_define): Fix
underquotation.
* data/skeletons/bison.m4 (b4_canary): New.
* tests/input.at (Code injection): Use it, and check lalr1.d too.

data/skeletons/location.cc

@@ -19,13 +19,6 @@ m4_pushdef([b4_copyright_years],
            [2002-2015, 2018-2021])
 
 
-# b4_position_file
-# ----------------
-# Name of the file containing the position class, if we want this file.
-b4_header_if([b4_required_version_if([30200], [],
-                                     [m4_define([b4_position_file], [position.hh])])])])
-
-
 # b4_location_file
 # ----------------
 # Name of the file containing the position/location class,
@@ -54,6 +47,16 @@ m4_ifdef([b4_location_file],
  ])
 
 
+# b4_position_file
+# ----------------
+# Name of the file containing the position class, if we want this file.
+b4_header_if(
+  [b4_required_version_if(
+    [30200], [],
+    [m4_ifdef([b4_location_file],
+              [m4_define([b4_position_file], [position.hh])])])])
+
+
 
 # b4_location_define
 # ------------------