diff --git a/docs/cookbook/c/.gitignore b/docs/cookbook/c/.gitignore
index 0703d8b..210b4a3 100644
--- a/docs/cookbook/c/.gitignore
+++ b/docs/cookbook/c/.gitignore
@@ -4,3 +4,4 @@ hello
 /arches
 /out
 /out.*
+/sources
diff --git a/docs/cookbook/c/all.rc.od b/docs/cookbook/c/all.rc.od
index 5b050d2..1ff6a57 100644
--- a/docs/cookbook/c/all.rc.od
+++ b/docs/cookbook/c/all.rc.od
@@ -1,6 +1,6 @@
 . ./redoconf.rc
 rc_include \
-	rc/CC.rc \
+	rc/CC.required.rc \
 	rc/CXX.rc \
 	rc/libqt4.rc \
 	rc/libgtk2.rc \
diff --git a/docs/cookbook/c/allconfig.do b/docs/cookbook/c/allconfig.do
index c659d30..807a1c9 100644
--- a/docs/cookbook/c/allconfig.do
+++ b/docs/cookbook/c/allconfig.do
@@ -8,9 +8,13 @@ config() {
 	(
 		cd "$dir" &&
 		../configure --host="$arch" "$@" &&
-		redo-ifchange rc/CC.rc &&
+		( set --;
+		  . ./redoconf.rc &&
+		  rc_include rc/CC.rc &&
+		  [ -n "$HAVE_CC" ]
+		) &&
 		echo "$dir"
-	) || :
+	) || (echo "Skipping arch '$arch' $*" >&2)
 }
 
 for d in $(cat arches); do
@@ -22,6 +26,6 @@ for d in $(cat arches); do
 	config "out.$d" "$arch" &
 	config "out.$d.static" "$arch" "--enable-static" &
 	config "out.$d.opt" "$arch" "--enable-optimization" &
-done
+done >$3
 
 wait
diff --git a/docs/cookbook/c/arches.do b/docs/cookbook/c/arches.do
index a779ebb..cb48e0a 100644
--- a/docs/cookbook/c/arches.do
+++ b/docs/cookbook/c/arches.do
@@ -1,14 +1,16 @@
-IFS=:
-echo native >$3
-if [ -z "$NO_SLOW_TESTS" ]; then
-	for dir in $PATH; do
-		for d in "$dir"/*-cc "$dir"/*-gcc; do
-			base=${d##*/}
-			arch=${base%-*}
-			if [ -x "$d" ]; then echo "$arch"; fi
+(
+	echo native
+	echo fake-always-fails
+	if [ -z "$NO_SLOW_TESTS" ]; then
+		IFS=:
+		for dir in $PATH; do
+			for d in "$dir"/*-cc "$dir"/*-gcc; do
+				base=${d##*/}
+				arch=${base%-*}
+				if [ -x "$d" ]; then echo "$arch"; fi
+			done
 		done
-	done >>$3
-fi
-
+	fi
+) >$3
 redo-always
 redo-stamp <$3
diff --git a/docs/cookbook/c/hello world.list.od b/docs/cookbook/c/hello world.list.od
index d9f5200..cb5548b 100644
--- a/docs/cookbook/c/hello world.list.od	
+++ b/docs/cookbook/c/hello world.list.od	
@@ -1,8 +1,8 @@
-# This script is run from the output dir,
-# which contains a src/ symlink to the source dir.
-
+# This script is run from the output dir.
+# The source dir is at $S.
 . ./redoconf.rc
 rc_include all.rc
+redo-ifchange "$S/sources"
 
 (
 	cd "$S"
@@ -26,5 +26,4 @@ rc_include all.rc
 	printf '%s\n' "$LIBGTK2" "$LIBQT4"
 ) >$3
 
-redo-always
 redo-stamp <$3
diff --git a/docs/cookbook/c/sources.do b/docs/cookbook/c/sources.do
new file mode 100644
index 0000000..81559c6
--- /dev/null
+++ b/docs/cookbook/c/sources.do
@@ -0,0 +1,12 @@
+# This file changes when the list of source files changes.
+# If you depend on it, you can make a target that gets
+# rebuilt only when it might need to reconsider the
+# list of available source files.
+find . -name '*.[ch]' -o \
+	-name '*.cc' -o \
+	-name '*.od' -o \
+	-name '*.list' |
+grep -v '^\./out\.' |
+sort >$3
+redo-always
+redo-stamp <$3
diff --git a/redoconf/rc.sh b/redoconf/rc.sh
index a5a3cd6..bc2fa4d 100644
--- a/redoconf/rc.sh
+++ b/redoconf/rc.sh
@@ -13,10 +13,10 @@ redo-ifchange "$REDOCONF/rc.sh" "$REDOCONF/utils.sh"
 _rc_exit_check() {
 	if [ -z "$RC_INCLUDE_RAN" ]; then
 		echo "Fatal: used redoconf/rc.sh but didn't call rc_include." >&2
-		exit 99
+		exit 91
 	elif [ -n "$RC_QUEUE" ]; then
 		echo "Fatal: must call rc_save or rc_undo before ending." >&2
-		exit 99
+		exit 92
 	fi
 }
 trap _rc_exit_check EXIT
diff --git a/redoconf/rc/CC.rc.od b/redoconf/rc/CC.rc.od
index 505ee6e..03605cd 100644
--- a/redoconf/rc/CC.rc.od
+++ b/redoconf/rc/CC.rc.od
@@ -57,7 +57,7 @@ for d in "$@"; do
 		x="${x#-}"
 		[ -n "$x" ] && x="$x-"
 		rc_replaceln ARCH "$x"
-
+		rc_replaceln HAVE_CC 1
 		rc_save
 		exit 0
 	fi
@@ -65,4 +65,5 @@ done
 
 echo "Can't find a working C compiler." >&2
 rc_undo
-exit 1
+rc_replaceln CC ""
+rc_save
diff --git a/redoconf/rc/CXX.rc.od b/redoconf/rc/CXX.rc.od
index 4859244..c6875c9 100644
--- a/redoconf/rc/CXX.rc.od
+++ b/redoconf/rc/CXX.rc.od
@@ -23,6 +23,7 @@ for d in "$@"; do
 		# it to include -lstdc++, etc.
 		# A future .rc could override this again.
 		rc_replaceln LINK "$d"
+		rc_replaceln HAVE_CXX 1
 		rc_save
 		exit 0
 	fi
@@ -32,4 +33,3 @@ echo "Warning: Can't find a working C++ compiler." >&2
 rc_undo
 rc_replaceln CXX ""
 rc_save
-exit 0
diff --git a/redoconf/rc/default.required.rc.od b/redoconf/rc/default.required.rc.od
new file mode 100644
index 0000000..157aee8
--- /dev/null
+++ b/redoconf/rc/default.required.rc.od
@@ -0,0 +1,13 @@
+. ./redoconf.rc
+
+base=${1%.required.rc}
+name=${base#*/}
+NAME=$(echo "$name" | tr 'a-z.' 'A-Z_')
+
+rc_include "$base.rc"
+
+eval v="\$$NAME"
+if [ -z "$v" ]; then
+	echo "$NAME is required in order to build." >&2
+	exit 1
+fi