Makefile: Add "rebuild-nodes" make target

Add "rebuild-nodes" make target to rebuild nodes for all levels that
match a specified pattern. Document "rebuild-nodes" in
BUILD-SYSTEM.adoc. Modify map06.wad so it adheres to the "no padding
between the lumps" constraint.

BUILD-SYSTEM.adoc

@@ -241,3 +241,29 @@ around.  Some of them are listed here.
    any +TEXTURE1+ lumps it generates.  To work around this, a “dummy”
    IWAD file containing an empty +TEXTURE1+ lump is contained inside
    the +dummy+ directory.
+
+== Node builder
+
+Each Freedoom level found in the "levels" directory should have exactly
+11 lumps in a https://zdoom.org/wiki/WAD[particular order] with no
+padding between the lumps. Of those 11 lumps 6 (<map name>, THINGS,
+LINEDEFS, SIDEDEFS, VERTEXES and SECTORS) are maintained by a level
+editor such as GZDoom Builder, SLADE or Eureka. The remaining 5 lumps
+(SEGS, SSECTORS, NODES, REJECT and BLOCKMAP) are added by a node
+builder. The Freedoom project uses the
+https://github.com/Doom-Utils/zennode[ZenNode] node builder. For
+example, to rebuild the nodes for all 100 levels run the
+"rebuild-nodes" make target in the top level makefile:
+[source,bash]
+-----------------
+make rebuild-nodes
+-----------------
+To control how the node builder is invoked, or which levels are
+rebuilt, override the NODE_BUILDER and NODE_BUILDER_LEVELS make
+variables respectively. For example, to specify the full path to
+ZenNode when it's not in the path and to rebuild only the first
+chapter of Freedoom 1 as well all of the deathmatch levels:
+[source,bash]
+-----------------
+make NODE_BUILDER=/opt/zennode/ZenNode NODE_BUILDER_LEVELS="c1m? dm??" rebuild-nodes
+-----------------

Makefile

@@ -9,6 +9,8 @@ CPP=scripts/simplecpp
 DEUTEX=deutex
 DEUTEX_BASIC_ARGS=-v0 -rate accept
 DEUTEX_ARGS=$(DEUTEX_BASIC_ARGS) -doom2 bootstrap/
+NODE_BUILDER=ZenNode
+NODE_BUILDER_LEVELS=c?m? dm?? map??
 
 FREEDOOM1=$(WADS)/freedoom1.wad
 FREEDOOM2=$(WADS)/freedoom2.wad
@@ -147,6 +149,15 @@ test: test-map-names
 fix-map-names:
 	scripts/fix-map-names levels
 
+# Rebuild the nodes for the level WADs. By default this invokes "ZenNode" on
+# all 100 level WADs. Override the "NODE_BUILDER" prefixed variables to
+# configure.
+rebuild-nodes: $(addprefix levels/,$(addsuffix .wad,$(NODE_BUILDER_LEVELS)))
+	for level in $^; \
+	do \
+		$(NODE_BUILDER) $$level -o $$level; \
+	done
+
 %.6:
 	$(MAKE) ASCIIDOC_MAN="$(ASCIIDOC_MAN)" -C dist $@