Improve how Makefile handles multiple goals.

- Don't run BUILDSYSTEM more than once if there is more than one goal
   with '+'.
 - Don't pass goals without '+' to BUILDSYSTEM.
 - Use $(MAKE) because "make" might not be GNU make.  For me, "make"
   is BSD make.
 - Add a comment so readers know MAKECMDGOALS is special.

Over in README, remove Lua from requirements; we always ignore any
installed Lua and build our own.  Increase guesses for free space
because we build more platforms.  Don't need to type MAKEFLAGS=.

Makefile

@@ -63,29 +63,50 @@ PLATDEP = $(INSDIR)/lib/ack
 
 .NOTPARALLEL:
 
-MAKECMDGOALS ?= +ack +tests
-BUILD_FILES = $(shell find * -name '*.lua')
-
-ifneq ($(shell which ninja),)
+ifeq ($(BUILDSYSTEM),)
+  ifneq ($(shell which ninja),)
 BUILDSYSTEM = ninja
-BUILDFLAGS = $(NINJAFLAGS)
-else
+  else
 BUILDSYSTEM = make
-BUILDFLAGS = $(MAKEFLAGS)
+  endif
 endif
 
-LUA = $(BUILDDIR)/lua
+build-file = $(BUILDDIR)/build.$(BUILDSYSTEM)
+lua-files = $(shell find * -name '*.lua')
+our-lua = $(BUILDDIR)/lua
 
-ifneq ($(findstring +, $(MAKECMDGOALS)),)
+# GNU make sets MAKECMDGOALS to the list of targets from the command
+# line.  We look for targets with '+' and forward them to BUILDSYSTEM.
+# This handles commands like
+#     $ make util/opt+pkg util/ego+pkg
 
-$(MAKECMDGOALS): $(BUILDDIR)/build.$(BUILDSYSTEM)
-	@$(BUILDSYSTEM) $(BUILDFLAGS) -f $^ $(MAKECMDGOALS)
+all-goals = +ack +tests
+plus-goals := $(patsubst all,$(all-goals),$(or $(MAKECMDGOALS),all))
+plus-goals := $(foreach g,$(plus-goals),$(if $(findstring +,$(g)),$(g),))
 
+# @true silences extra message, "make: Nothing to be done..."
+
+all: build-plus-goals
+	@true
+
+ifneq ($(plus-goals),)
+$(plus-goals): build-plus-goals
+	@true
 endif
 
-$(BUILDDIR)/build.$(BUILDSYSTEM): first/ackbuilder.lua Makefile $(BUILD_FILES) $(LUA)
+build-plus-goals: $(build-file)
+ifeq ($(BUILDSYSTEM),ninja)
+	@ninja $(NINJAFLAGS) -f $(build-file) $(plus-goals)
+else ifeq ($(BUILDSYSTEM),make)
+# GNU make passes MAKEFLAGS in environment.
+	@$(MAKE) -f $(build-file) $(plus-goals)
+else
+$(error unknown BUILDSYSTEM = $(BUILDSYSTEM))
+endif
+
+$(build-file): first/ackbuilder.lua Makefile $(lua-files) $(our-lua)
 	@mkdir -p $(BUILDDIR)
-	@$(LUA) first/ackbuilder.lua \
+	@$(our-lua) first/ackbuilder.lua \
 		first/build.lua build.lua \
 		--$(BUILDSYSTEM) \
 		DEFAULT_PLATFORM=$(DEFAULT_PLATFORM) \
@@ -99,17 +120,16 @@ $(BUILDDIR)/build.$(BUILDSYSTEM): first/ackbuilder.lua Makefile $(BUILD_FILES) $
 		PREFIX=$(PREFIX) \
 		AR=$(AR) \
 		CC=$(CC) \
-		> $(BUILDDIR)/build.$(BUILDSYSTEM)
+		> $(build-file)
 
 install:
 	mkdir -p $(PREFIX)
 	tar cf - -C $(INSDIR) . | tar xvf - -C $(PREFIX)
 
 clean:
-	@rm -rf $(BUILDDIR)
+	rm -rf $(BUILDDIR)
 
-$(LUA): first/lua-5.1/*.c first/lua-5.1/*.h
+$(our-lua): first/lua-5.1/*.c first/lua-5.1/*.h
 	@echo Bootstrapping build
 	@mkdir -p $(BUILDDIR)
-	@$(CC) -o $(LUA) -O first/lua-5.1/*.c -lm
-
+	@$(CC) -o $(our-lua) -O first/lua-5.1/*.c -lm

README

@@ -53,8 +53,6 @@ Requirements:
 
 - GNU make.
 
-- Lua 5.1 and the luaposix library (used by the build system).
-
 - (optionally) ninja; if you've got this, this will be autodetected and give
   you faster builds.
 
@@ -63,9 +61,9 @@ Requirements:
   architectures. Get both the qemu-system-* platform emulators and the qemu-*
   userland emulators (only works on Linux).
 
-- about 40MB free in /tmp (or some other temporary directory).
+- about 115MB free in /tmp (or some other temporary directory).
 
-- about 6MB in the target directory.
+- about 15MB in the target directory.
 
 Instructions:
 
@@ -83,7 +81,7 @@ Instructions:
   install ninja and it'll use all your cores. If you don't have ninja, you
   can still use make for parallel builds with:
 
-    make MAKEFLAGS='-r -j8'   # or however many cores you have
+    make -r -j8   # or however many cores you have
 
   ...but frankly, I recommend ninja.