style: rename spec_defines_file as spec_header_file

The variable spec_defines_file denotes the name of the generated
header.  Its name is derived from --defines/%defines, whose name in
turn is derived from the fact that the header, in Yacc, contained the

Not only does the header now contain a lot more than just the token
definitions, but we no longer even generate macros, but an enum...

Let's modernize our vocabulary.

* src/files.h, src/files.c (spec_defines_file): Rename as...
(spec_header_file): this.

src/output.c

@@ -679,7 +679,7 @@ prepare (void)
 #define DEFINE(Name) MUSCLE_INSERT_STRING (#Name, Name ? Name : "")
   DEFINE (dir_prefix);
   DEFINE (parser_file_name);
-  DEFINE (spec_defines_file);
+  DEFINE (spec_header_file);
   DEFINE (spec_file_prefix);
   DEFINE (spec_graph_file);
   DEFINE (spec_name_prefix);