M4: generate synchronization lines

This also includes Bison version check. Versions before 3.0 don't
support them in a reliable way and we don't promise to work with
versions older than 2.4.

Makefile.in

@@ -9,6 +9,7 @@ MAKEFLAGS += -r
 CPPFLAGS=-I$(objdir) -I$(srcdir) @CPPFLAGS@
 CFLAGS=$(CPPFLAGS) @CFLAGS@
 LDFLAGS=@LDFLAGS@
+M4FLAGS=@M4FLAGS@
 LIBS=@LIBS@
 DAEMON_LIBS=@DAEMON_LIBS@
 CLIENT_LIBS=@CLIENT_LIBS@

aclocal.m4

@@ -175,3 +175,22 @@ AC_DEFUN([BIRD_CHECK_PROG_FLAVOR_GNU],
   )
   esac
 ])
+
+AC_DEFUN([BIRD_CHECK_BISON_VERSION],
+[
+  $1=`bison --version | ( read line; echo ${line##* } )`
+  case "$$1" in
+    1.* | 2.0* | 2.1* | 2.2* | 2.3*)
+      AC_MSG_ERROR([Provided Bison version $$1 is too old, need at least 2.4])
+      ;;
+    2.*)
+      bird_bison_synclines=no
+      ;;
+    3.* | 4.* | 5.* | 6.* | 7.* | 8.* | 9.*)
+      bird_bison_synclines=yes
+      ;;
+    *)
+      AC_MSG_ERROR([Couldn't parse Bison version $$1. Call the developers for help.])
+      ;;
+  esac
+])

conf/Makefile

@@ -11,7 +11,7 @@ BISON_DEBUG=-t
 endif
 
 $(conf-y-targets): $(s)confbase.Y $(s)flowspec.Y
-	$(M4) -P $| $^ >$@
+	$(M4) $(M4FLAGS) -P $| $^ >$@
 
 $(o)cf-parse.y: | $(s)gen_parser.m4
 $(o)keywords.h: | $(s)gen_keywords.m4

configure.ac

@@ -157,6 +157,15 @@ test -z "$FLEX"	 && AC_MSG_ERROR([Flex is missing.])
 test -z "$BISON" && AC_MSG_ERROR([Bison is missing.])
 test -z "$M4"	 && AC_MSG_ERROR([M4 is missing.])
 
+AC_MSG_CHECKING([bison version])
+BIRD_CHECK_BISON_VERSION(BISON_VERSION)
+AC_MSG_RESULT([$BISON_VERSION])
+if test "$bird_bison_synclines" = yes; then
+  M4FLAGS="$M4FLAGS -s"
+fi
+
+AC_SUBST([M4FLAGS])
+
 BIRD_CHECK_PROG_FLAVOR_GNU([$M4],
   [],
   [AC_MSG_ERROR([Provided M4 is not GNU M4.])]