opencv2: don't build unfree libraries by default

In opencv 2.x, unfree libraries are built by default. The package
should therefore have been marked as unfree, but wasn't.

I've disabled the non-free libraries by default, and added an option
to enable them. There are three programs in Nixpkgs that depend on
opencv2: mathematica, pfstools, and p2pvc. pfstools requires the
non-free libraries if it's built with opencv support, so I've disabled
opencv by default there and added an option to enable it. p2pvc links
fine, so presumably doesn't need the non-free libraries. I can't test
mathematica, so I'm just going to leave it alone.

nixos/doc/manual/from_md/release-notes/rl-2111.section.xml

···
           changelog</link>.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          <literal>opencv2</literal> no longer includes the non-free
+          libraries by default, and consequently
+          <literal>pfstools</literal> no longer includes OpenCV support
+          by default. Both packages now support an
+          <literal>enableUnfree</literal> option to re-enable this
+          functionality.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
   <section xml:id="sec-release-21.11-notable-changes">

nixos/doc/manual/release-notes/rl-2111.section.md

···
   respectively. As a result `services.datadog-agent` has had breaking changes to the
   configuration file. For details, see the [upstream changelog](https://github.com/DataDog/datadog-agent/blob/main/CHANGELOG.rst).
 
+- `opencv2` no longer includes the non-free libraries by default, and consequently `pfstools` no longer includes OpenCV support by default.  Both packages now support an `enableUnfree` option to re-enable this functionality.
+
 ## Other Notable Changes {#sec-release-21.11-notable-changes}
 
 

pkgs/development/libraries/opencv/default.nix

···
 , enableFfmpeg ? false, ffmpeg
 , enableGStreamer ? false, gst_all_1
 , enableEigen ? true, eigen
+, enableUnfree ? false
 , Cocoa, QTKit
 }:
 
···
     (opencvFlag "PNG" enablePNG)
     (opencvFlag "OPENEXR" enableEXR)
     (opencvFlag "GSTREAMER" enableGStreamer)
-  ];
+  ] ++ lib.optional (!enableUnfree) "-DBUILD_opencv_nonfree=OFF";
 
   hardeningDisable = [ "bindnow" "relro" ];
 
···
   meta = with lib; {
     description = "Open Computer Vision Library with more than 500 algorithms";
     homepage = "https://opencv.org/";
-    license = licenses.bsd3;
+    license = if enableUnfree then licenses.unfree else licenses.bsd3;
     maintainers = with maintainers; [ ];
     platforms = platforms.linux;
   };

pkgs/tools/graphics/pfstools/default.nix

···
 { lib, stdenv, mkDerivation, fetchurl, cmake, pkg-config, darwin
 , openexr, zlib, imagemagick6, libGLU, libGL, freeglut, fftwFloat
-, fftw, gsl, libexif, perl, opencv2, qtbase, netpbm
+, fftw, gsl, libexif, perl, qtbase, netpbm
+, enableUnfree ? false, opencv2
 }:
 
 mkDerivation rec {
···
   nativeBuildInputs = [ cmake pkg-config ];
   buildInputs = [
     openexr zlib imagemagick6 fftwFloat
-    fftw gsl libexif perl opencv2 qtbase netpbm
+    fftw gsl libexif perl qtbase netpbm
   ] ++ (if stdenv.isDarwin then (with darwin.apple_sdk.frameworks; [
     OpenGL GLUT
   ]) else [
     libGLU libGL freeglut
-  ]);
+  ]) ++ lib.optional enableUnfree (opencv2.override { enableUnfree = true; });
 
   patches = [ ./threads.patch ./pfstools.patch ./pfsalign.patch ];