sct: fix segfault when DISPLAY is empty, clean up derivation

pkgs/tools/X11/sct/DISPLAY-segfault.patch

···
+--- a/sct.c	2017-09-22 00:44:20.270421881 +0000
++++ b/sct.c	2017-09-26 10:50:38.964562740 +0000
+@@ -36,6 +36,7 @@
+ main(int argc, char **argv)
+ {
+ 	Display *dpy = XOpenDisplay(NULL);
++	if (!dpy) exit(1);
+ 	int screen = DefaultScreen(dpy);
+ 	Window root = RootWindow(dpy, screen);
+ 

pkgs/tools/X11/sct/default.nix

···
-{stdenv, fetchurl, libX11, libXrandr}:
+{ stdenv, fetchurl, libX11, libXrandr }:
+
 stdenv.mkDerivation rec {
   name = "sct";
-  buildInputs = [libX11 libXrandr];
+
   src = fetchurl {
     url = http://www.tedunangst.com/flak/files/sct.c;
     sha256 = "01f3ndx3s6d2qh2xmbpmhd4962dyh8yp95l87xwrs4plqdz6knhd";
-    
-    # Discussion regarding the checksum and the source code can be found in issue #17163 
-    # The code seems unmaintained, yet an unknown (probably small change) in the code caused 
-    # failed builds as the checksum had changed.
-    # The checksum is updated for now, however, this is unpractical and potentially unsafe 
-    # so any future changes might warrant a fork of the (feature complete) project. 
-    # The code is under public domain.
-    
   };
-  phases = ["patchPhase" "buildPhase" "installPhase"];
-  patchPhase = ''
-    sed -re "/Xlibint/d" ${src} > sct.c 
-  '';
-  buildPhase = "gcc -std=c99 sct.c -o sct -lX11 -lXrandr -lm";
-  installPhase = ''
-    mkdir -p "$out/bin"
-    cp sct "$out/bin"
-  '';
-  meta = {
-    description = ''A minimal utility to set display colour temperature'';
-    maintainers = [stdenv.lib.maintainers.raskin];
-    platforms = with stdenv.lib.platforms; linux ++ freebsd ++ openbsd;
+
+  unpackPhase = "cat ${src} > sct.c";
+  patches = [ ./DISPLAY-segfault.patch ];
+
+  buildInputs = [ libX11 libXrandr ];
+  buildPhase = "cc sct.c -o sct -lm -lX11 -lXrandr";
+
+  installPhase = "install -Dt $out/bin sct";
+
+  meta = with stdenv.lib; {
+    homepage = https://www.tedunangst.com/flak/post/sct-set-color-temperature;
+    description = "A minimal utility to set display colour temperature";
+    maintainers = [ maintainers.raskin ];
+    license = licenses.publicDomain;
+    platforms = with platforms; linux ++ freebsd ++ openbsd;
   };
 }