docs/cookbook/redoconf-simple: a simple redoconf C++ project.

This is a little simpler than the docs/cookbook/c project, which
doesn't actually have a doc yet because there was too much to explain.
I think I might make that a follow-on cookbook chapter, for people who
have read this simple one.

I think this doc is maybe a little too long; I intended it to be
"here's what you do to get started" but it turned into "here's what you
do to get started, and why it works, in excruciating detail." Not quite
sure how to fix.

(Also updated some other parts of the docs to refer to redoconf as a
real thing now instead of a "maybe someone should write this" thing.)

docs/cookbook/redoconf-simple/skip-if-no-cxx.sh

@@ -0,0 +1,14 @@
+# Make sure there is an out/ directory and
+# the configure script was run.
+redo-ifchange configured
+
+# Abort if we can't find a C++ compiler for
+# this platform.
+if ! (cd out &&
+      . ./redoconf.rc &&
+      rc_include rc/CXX.rc &&
+      [ -n "$HAVE_CXX" ]); then
+	echo "$1: No C++ compiler: skipping." >&2
+	exit 0
+fi
+