* tests/calc.at: Exercise prologue splitting.

* data/bison.simple, data/bison.c++: Use `b4_pre_prologue' and
`b4_post_prologue' instead of `b4_prologue'.
* src/output.c (prepare): Add the `pre_prologue' and `post_prologue'
muscles.
(output): Free pre_prologue_obstack and post_prologue_obstack.
* src/files.h, src/files.c (attrs_obstack): Remove.
(pre_prologue_obstack, post_prologue_obstack): New.
* src/reader.c (copy_definition): Add a parameter to specify the
obstack to fill, instead of using attrs_obstack unconditionally.
(read_declarations): Pass pre_prologue_obstack to copy_definition if
`%union' has not yet been seen, pass post_prologue_obstack otherwise.

data/bison.c++

@@ -113,7 +113,8 @@ b4_copyright
 /* Using locations.  */
 #define YYLSP_NEEDED b4_locations_flag
 
-b4_prologue
+/* Copy the first part of user declarations.  */
+b4_pre_prologue
 
 /* Tokens.  */
 b4_token_defines(b4_tokens)
@@ -137,6 +138,9 @@ yystype;
 # define YYSTYPE yystype
 #endif
 
+/* Copy the second part of user declarations.  */
+b4_post_prologue
+
 #line __oline__  "__ofile__"
 #ifndef YYLLOC_DEFAULT
 # define YYLLOC_DEFAULT(Current, Rhs, N) \

data/bison.simple

@@ -120,8 +120,8 @@ m4_if(b4_prefix[], [yy], [],
 #endif
 ])
 
-/* Copy the user declarations.  */
-b4_prologue
+/* Copy the first part of user declarations.  */
+b4_pre_prologue
 
 /* Enabling traces.  */
 #ifndef YYDEBUG
@@ -155,6 +155,9 @@ typedef struct yyltype
 # define YYLTYPE b4_ltype
 #endif
 
+/* Copy the second part of user declarations.  */
+b4_post_prologue
+
 /* Line __line__ of __file__.  */
 #line __oline__ "__ofile__"