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.

CMakeLists.txt

@@ -61,7 +61,8 @@ endif ()
 
 # -lrt is only for glibc < 2.17
 # -liconv may or may not be a part of libc
-foreach (extra iconv rt)
+# -lm may or may not be a part of libc
+foreach (extra iconv rt m)
 	find_library (extra_lib_${extra} ${extra})
 	if (extra_lib_${extra})
 		list (APPEND project_libraries ${extra_lib_${extra}})