Bison 1.50 pre & post prologue bug

[Hans Aberg]

Bison 1.50 does not seem to handle the pre & post prologues properly; that
is, when writing:
%{
  pre_prologue
%}
...
%{
  post_prologue
%}
...
%%
...
in the .y files as above, the b4_post_prologue macro expands to empty, and
all stuff goes into the first b4_pre_prologue macro.

-- I need this for writing my C++ skeleton file: Some stuff should then go
into the header and other into the source file. This is often required with
polymorphic classes and stuff relying on forward declarations, but also
useful in order to decrease the number of headers that the header includes.

  Hans Aberg