Add configuration check for setlinebuf

This function is not present on MinGW (of course), and
the equivalent behaves in a non-sensical way (full buffering).

See discussion here: https://stackoverflow.com/questions/15972550/what-is-the-use-of-setvbuf-in-an-applicationexecutable

config/check_setlinebuf.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+
+int main()
+{
+    FILE *f = stdout;
+    setlinebuf(f);
+}

example/Makefile

@@ -51,6 +51,7 @@ CONFIG=	<stdio.h>		\
 	<sys/improbable.h> 	\
 	<iostream>		\
 	clearenv		\
+	setlinebuf		\
 	libm			\
 	liboony			\
 	sbrk