opencv4: expose cxxdev, propagating optional cuda deps

pkgs/development/libraries/opencv/4.x.nix

@@ -247,8 +247,10 @@ effectiveStdenv.mkDerivation {
 
   outputs = [
     "out"
+    "cxxdev"
     "package_tests"
   ];
+  cudaPropagateToOutput = "cxxdev";
 
   postUnpack = lib.optionalString buildContrib ''
     cp --no-preserve=mode -r "${contribSrc}/modules" "$NIX_BUILD_TOP/source/opencv_contrib"
@@ -328,7 +330,7 @@ effectiveStdenv.mkDerivation {
       bzip2 AVFoundation Cocoa VideoDecodeAcceleration CoreMedia MediaToolbox Accelerate
     ]
     ++ lib.optionals enableDocs [ doxygen graphviz-nox ]
-    ++ lib.optionals enableCuda  (with cudaPackages; [
+    ++ lib.optionals enableCuda (with cudaPackages; [
       cuda_cudart
       cuda_cccl # <thrust/*>
       libnpp # npp.h
@@ -338,7 +340,7 @@ effectiveStdenv.mkDerivation {
       cudnn # cudnn.h
     ] ++ lib.optionals enableCufft [
       libcufft # cufft.h
-  ]);
+    ]);
 
   propagatedBuildInputs = lib.optional enablePython pythonPackages.numpy
     ++ lib.optionals enableCuda [ nvidia-optical-flow-sdk ];
@@ -458,6 +460,7 @@ effectiveStdenv.mkDerivation {
   postInstall = ''
     sed -i "s|{exec_prefix}/$out|{exec_prefix}|;s|{prefix}/$out|{prefix}|" \
       "$out/lib/pkgconfig/opencv4.pc"
+    mkdir $cxxdev
   ''
   # install python distribution information, so other packages can `import opencv`
   + lib.optionalString enablePython ''