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/bison.m4

@@ -261,6 +261,13 @@ m4_define([b4_fatal_at],
 [b4_error([[fatal]], $@)dnl
 m4_exit(1)])
 
+# b4_canary(MSG)
+# --------------
+# Issue a warning on stderr and in the output.  Used in the test suite
+# to catch spurious m4 evaluations.
+m4_define([b4_canary],
+[m4_errprintn([dead canary: $1])DEAD CANARY($1)])
+
 
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