Implement --trace=m4.

* src/getargs.c (trace_types, trace_args): Accept trace_m4.
* src/output.c (output_skeleton): When set, pass -dV to m4.
Factor the handling of flags in m4.
* src/output.c (prepare): Rename the muscle names debug, defines,
error_verbose to debug_flag, defines_flag, error_verbose_flag.
* data/c.m4: Adjust.
(_b4_define_flag_if, b4_define_flag_if, b4_defines_if): New.
Use b4_define_flag_if to define other b4_FLAG_if macros.
(b4_location_if): As a consequence, rename as...
(b4_locations_if): this, for consistency.
Adjust all the skeletons.

src/getargs.c

@@ -84,7 +84,8 @@ static const char * const trace_args[] =
   "grammar    - reading, reducing of the grammar",
   "resource   - memory consumption (where available)",
   "sets       - grammar sets: firsts, nullable etc.",
-  "tools      - m4 invocation and preserve the temporary file",
+  "tools      - m4 invocation",
+  "m4         - m4 traces",
   "skeleton   - skeleton postprocessing",
   "time       - time consumption",
   "all        - all of the above",
@@ -102,6 +103,7 @@ static const int trace_types[] =
   trace_resource,
   trace_sets,
   trace_tools,
+  trace_m4,
   trace_skeleton,
   trace_time,
   trace_all