build: don't require src/bison during bootstrap.

Suggested by Eric Blake at
<http://lists.gnu.org/archive/html/bug-bison/2010-03/msg00003.html>.
* bootstrap.conf (bootstrap_epilogue): New function to make sure
src/parse-gram.[ch] are stamped later than src/parse-gram.y.

bootstrap.conf

@@ -65,3 +65,10 @@ excluded_files='
 # In directories like lib/, m4/, and po/, generate both these files because we
 # still try to support CVS emulation.
 vc_ignore='.cvsignore .gitignore'
+
+bootstrap_epilogue()
+{
+  # Make sure we don't need src/bison, which usually doesn't exist at
+  # the time of a bootstrap.
+  touch src/parse-gram.[ch]
+}