redoconf: better handling of required vs optional detectors.

CC.rc was the only "mandatory" detection, which was weird and
inconsistent.  Instead, make it optional like the others, and have it
set a HAVE_CC variable appropriately (and have CXX.rc work the same
way).  Then, add a default.required.rc.od that checks the HAVE_* for
any variable and aborts if it is not available.

This allows us to fix confusing behaviour in allconfig.do, which would
try every compiler on the system, but redo would print a (non-fatal)
error message (and prevent redo-stamp optimization) when CC.rc failed
for any non-working compilers.  Now CC.rc just politely reports that it
didn't find a compiler.  Then we change all.rc.od to make CC.rc
mandatory.

Reported-by: Nathaniel Filardo <nwfilardo@gmail.com>

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