Digging up my old "make standalone" stuff from a year ago:

http://busybox.net/lists/busybox/2005-September/015766.html

I renamed it "individual" to not confuse it with the standalone shell.  (Which
it isn't compatible with for obvious reasons.)  Configure busybox (I did
make defconfig), then run scripts/individual and it'll build an individual
version of each applet in the "build" subdirectory.

Currently it builds 146 and fails to build 104 applets out of "make defconfig".
I haven't taught it about multi-file applets yet (like tar), or the ones where
two applets get built from the same source (for example, zcat is a trivial
variant of gunzip so there is no zcat.c).  But here's a start.

applets/individual.c

@@ -0,0 +1,26 @@
+/* Minimal wrapper to build an individual busybox applet.
+ *
+ * Copyright 2005 Rob Landley <rob@landley.net
+ * 
+ * Licensed under GPLv2 or later, see file License in this tarball for details
+ */
+
+const char *bb_applet_name;
+
+#include <stdio.h>
+#include <stdlib.h>
+#include "usage.h"
+
+int main(int argc, char *argv[])
+{
+	bb_applet_name=argv[0];
+
+	return APPLET_main(argc,argv);
+}
+
+void bb_show_usage(void)
+{
+	printf(APPLET_full_usage "\n");
+
+	exit(1);
+}