get retval(1) working again

libio/Makefile

@@ -0,0 +1,2 @@
+libio.o: libio.c libio.h
+	$(CC) -c -o libio.o libio.c

libio/libio.c

@@ -3,19 +3,11 @@
 
 /* Backups of standard library systems for the odd scenario where there is no
  * standard library but there is the write(2) system call. */
-static int _isdigit(int c);
-static int _strlen(char *s);
-
-/* Wrappers around write(2) */
-static int _write(int fd, char *s, int n);
-static int _write1(int fd, int c);
-
-/* Normally in <ctype.h> */
-static int
-_isdigit(int c){
-	return c >= '0' && c <= '9';
-}
 
+#if defined USE_SYSTEM_STRING_H
+#	include <string.h>
+#	define _strlen strlen
+#else
 /* Normally in <string.h> */
 static int
 _strlen(char *s){
@@ -25,6 +17,22 @@ _strlen(char *s){
 
 	return i;
 }
+#endif
+
+#if defined USE_SYSTEM_CTYPE_H
+#	include <ctype.h>
+#	define _isdigit isdigit
+#else
+/* Normally in <ctype.h> */
+static int
+_isdigit(int c){
+	return c >= '0' && c <= '9';
+}
+#endif
+
+/* Wrappers around write(2) */
+static int _write(int fd, char *s, int n);
+static int _write1(int fd, int c);
 
 /* Wrapper around write(2) - keeps re-running write(2) until the operation
  * succeeds in full. Usually unnecessary but still good. */
@@ -52,7 +60,7 @@ _write1(int fd, int c){
 
 int
 fdprint(int fd, char *s){
-	return _write(fd, s, strlen(s));
+	return _write(fd, s, _strlen(s));
 }
 
 int