api.prefix: also rename YYDEBUG.

api.prefix: also rename YYDEBUG.

The testsuite in master has shown weird errors for the "Mulitple
Parsers" tests: the caller of p5.parse() received some apparently
random value, while tracing p5.parse() showed that the function was
consistently returning 0.

It happens when mixing several parser headers, some generated without
%debug, others with.  In particular the C++ parser was generated with
%debug, i.e., with:

  #ifndef YYDEBUG
  # define YYDEBUG 1
  #endif

and compiled separatedly.  Yet, its header was included after the one
of another parser, this time without %debug, i.e., with

  #ifndef YYDEBUG
  # define YYDEBUG 0
  #endif

in its header.  As a result, the parser was compiled with YYDEBUG set,
but its header was used without.  Since the layout of the objects are
then completely different, boom.

Therefore, do not change the value of YYDEBUG.  Rather, use it as a
default value for <API.PREFIX>DEBUG.

* data/c.m4 (b4_YYDEBUG_define): New.
(b4_declare_yydebug): Rename as...
(b4_yydebug_declare): this, for consistency.
* data/glr.c, data/glr.cc, data/lalr1.cc, data/yacc.c: Use it.
* NEWS: Document it.