degesch: pick colours based on relative luminance

Replaces the inaccurate Rec. 709 luma we used to use before.

This is the first feature here that requires libm, which doesn't
seem to be a particularly great sacrifice.

Moreover, I've rectified that the input isn't linear in sRGB,
and then was even normalized wrong for the luma formula.

test-nick-colors

@@ -0,0 +1,23 @@
+#!/bin/sh
+# Check whether the terminal colours filtered by our algorithm are legible
+export example=$(
+	tcc "-run -lm" - <<-EOF
+	#include <stddef.h>
+	#include <stdio.h>
+	#include <math.h>
+
+	$(perl -0777 -ne 'print $& if /^.*?\nfilter_color(?s:.*?)^}$/m' degesch.c)
+
+	void main () {
+		size_t len = 0;
+		int *table = filter_color_cube_for_acceptable_nick_colors (&len);
+		for (size_t i = 0; i < len; i++)
+			printf ("<@\\x1b[38;5;%dmIRCuser\\x1b[m> I'm typing!\n", table[i]);
+	}
+	EOF
+)
+
+# Both should give acceptable results,
+# which results in a bad compromise that the main author himself needs
+xterm -bg black -fg white -e 'echo $example; cat' &
+xterm -bg white -fg black -e 'echo $example; cat' &