docs/cookbook/c/allconfig.do: avoid need for '&' backgrounding.

Instead of running a bunch of separate rc_include statements in the
background, which causes unpredictable ordering of log output and
prevents -j from controlling parallelism, let's do a single
redo-ifchange for all of them (the slow part) followed by sequentially
checking the results (the fast part).

docs/cookbook/c/allconfig.do

@@ -8,24 +8,35 @@ config() {
 	(
 		cd "$dir" &&
 		../configure --host="$arch" "$@" &&
-		( set --;
-		  . ./redoconf.rc &&
-		  rc_include rc/CC.rc &&
-		  [ -n "$HAVE_CC" ]
-		) &&
 		echo "$dir"
-	) || (echo "Skipping arch '$arch' $*" >&2)
+	)
 }
 
-for d in $(cat arches); do
+dirs=$(
+	for d in $(cat arches); do
 		if [ "$d" = "native" ]; then
 			arch=""
 		else
 			arch="$d"
 		fi
-	config "out.$d" "$arch" &
-	config "out.$d.static" "$arch" "--enable-static" &
-	config "out.$d.opt" "$arch" "--enable-optimization" &
+		config "out.$d" "$arch"
+		config "out.$d.static" "$arch" "--enable-static"
+		config "out.$d.opt" "$arch" "--enable-optimization"
+	done
+)
+
+for dir in $dirs; do
+	echo "$dir/rc/CC.rc"
+done | xargs redo-ifchange
+
+for dir in $dirs; do
+	( cd "$dir" &&
+	  set --;
+	  . ./redoconf.rc &&
+	  rc_include rc/CC.rc &&
+	  [ -n "$HAVE_CC" ] &&
+	  echo "$dir"
+	) || (echo "Skipping $dir' - no working C compiler." >&2)
 done >$3
 
 wait