* data/glr.c (b4_shared_declarations): Put start-header first,

before any #includes that we generate, so that feature-test
macros work.  Problem reported by Michael Deutschmann in
<http://lists.gnu.org/archive/html/bug-bison/2006-09/msg00004.html>.
* data/lalr1.cc: Likewise.
* doc/bison.texinfo (Prologue): Document that feature-test macros
should be defined before any Bison declarations.
* tests/actions.at (_AT_CHECK_PRINTER_AND_DESTRUCTOR): Put defns
that depend on location.hh after, not before, Bison decls, since
we now include location.hh after the first user prologue.

doc/bison.texinfo

@@ -2649,6 +2649,7 @@ can be done with two @var{Prologue} blocks, one before and one after the
 
 @smallexample
 %@{
+  #define _GNU_SOURCE
   #include <stdio.h>
   #include "ptypes.h"
 %@}
@@ -2666,6 +2667,13 @@ can be done with two @var{Prologue} blocks, one before and one after the
 @dots{}
 @end smallexample
 
+When in doubt, it is usually safer to put prologue code before all
+Bison declarations, rather than after.  For example, any definitions
+of feature test macros like @code{_GNU_SOURCE} or
+@code{_POSIX_C_SOURCE} should appear before all Bison declarations, as
+feature test macros can affect the behavior of Bison-generated
+@code{#include} directives.
+
 @findex %before-header
 @findex %start-header
 @findex %after-header