yacc.c: provide a means to include the header in the implementation

Currently when --defines is used, we generate a header, and paste an
exact copy of it into the generated parser implementation file.  Let's
provide a means to #include it instead.

We don't do it by default because of the Autotools' ylwrap.  This
program wraps invocations of yacc (that uses a fixed output name:
y.tab.c, y.tab.h, y.output) to support a more modern naming
scheme (dir/foo.y -> dir/foo.tab.c, dir/foo.tab.h, etc.).  It does
that by renaming the generated files, and then by running sed to
propagate these renamings inside the files themselves.

Unfortunately Automake's Makefiles uses Bison as if it were Yacc (with
--yacc or with -o y.tab.c) and invoke bison via ylwrap.  As a
consequence, as far as Bison is concerned, the output files are
y.tab.c and y.tab.h, so it emits '#include "y.tab.h"'.  So far, so
good.  But now ylwrap processes this '#include "y.tab.h"' into
'#include "dir/foo.tab.h"', which is not guaranteed to always work.

So, let's do the Right Thing when the output file is not y.tab.c, in
which case the user should %define api.header.include.  Binding this
behavior to --yacc is tempting, but we recently told people to stop
using --yacc (as it also enables the Yacc warnings), but rather to use
-o y.tab.c.

Yacc.c is the only skeleton concerned: all the others do include their
header.

* data/skeletons/yacc.c (b4_header_include_if): New.
(api.header.include): Provide a default value when the output is not
y.tab.c.
* src/parse-gram.y (api.header.include): Define.

NEWS

@@ -15,6 +15,24 @@ GNU Bison NEWS
   When given -fsyntax-only, the diagnostics are reported, but no output is
   generated.
 
+*** Include the generated header (yacc.c)
+
+  Before, when --defines is used, bison generated a header, and pasted an
+  exact copy of it into the generated parser implementation file.  If the
+  header name is not "y.tab.h", it is now #included instead of being
+  duplicated.
+
+  To use an '#include' even if the header name is "y.tab.h" (which is what
+  happens with --yacc, or when using the Autotools' ylwrap), define
+  api.header.include to the exact argument to pass to #include.  For
+  instance:
+
+    %define api.header.include {"parse.h"}
+
+  or
+
+    %define api.header.include {<parser/parse.h>}
+
 ** Documentation
 
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