waifu2x-converter-cpp: Enable build on Darwin

pkgs/tools/graphics/waifu2x-converter-cpp/default.nix

···
-{ cmake, fetchFromGitHub, makeWrapper, opencv3, lib, stdenv, ocl-icd, opencl-headers
+{ cmake, fetchFromGitHub, makeWrapper, opencv3, lib, stdenv, ocl-icd, opencl-headers, OpenCL
 , cudaSupport ? false, cudatoolkit ? null
 }:
 
···
     sha256 = "0rv8bnyxz89za6gwk9gmdbaf3j7c1j52mip7h81rir288j35m84x";
   };
 
+  patches = [
+    # Remove the hard-coded compiler on Darwin and use the one in stdenv.
+    ./waifu2x_darwin_build.diff
+  ];
+
   buildInputs = [
-    ocl-icd opencv3 opencl-headers
-  ] ++ lib.optional cudaSupport cudatoolkit;
+    opencv3
+  ] ++ lib.optional cudaSupport cudatoolkit
+    ++ lib.optional stdenv.isDarwin OpenCL
+    ++ lib.optionals stdenv.isLinux [ ocl-icd opencl-headers ];
 
   nativeBuildInputs = [ cmake makeWrapper ];
 
-  preFixup = ''
+  preFixup = lib.optionalString stdenv.isLinux ''
     wrapProgram $out/bin/waifu2x-converter-cpp --prefix LD_LIBRARY_PATH : "${ocl-icd}/lib"
   '';
 
···
     homepage = "https://github.com/DeadSix27/waifu2x-converter-cpp";
     license = lib.licenses.mit;
     maintainers = [ lib.maintainers.xzfc ];
-    platforms = lib.platforms.linux;
+    platforms = lib.platforms.linux ++ lib.platforms.darwin;
   };
 }

pkgs/tools/graphics/waifu2x-converter-cpp/waifu2x_darwin_build.diff

···
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index 8b728b5..dedf8b6 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -71,10 +71,6 @@ message(STATUS "System is: ${CMAKE_SYSTEM_NAME} (${LOCAL_SYS_TYPE})")
+ 
+ ### Get binary paths for APPLE users
+ if(APPLE AND CMAKE_CXX_COMPILER_ID MATCHES "^(Apple)?Clang$")
+-	set(CMAKE_C_COMPILER "/usr/local/opt/llvm/bin/clang")
+-	set(CMAKE_CXX_COMPILER "/usr/local/opt/llvm/bin/clang++")
+-	set(CMAKE_EXE_LINKER_FLAGS "-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib")
+-	set(CMAKE_SHARED_LINKER_FLAGS "-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib")
+ 	if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9.0)
+ 		set(FILE_SYSTEM_LIB "c++fs")
+ 	endif()

pkgs/top-level/all-packages.nix

···
     openssl = openssl_1_0_2;
   };
 
-  waifu2x-converter-cpp = callPackage ../tools/graphics/waifu2x-converter-cpp { };
+  waifu2x-converter-cpp = callPackage ../tools/graphics/waifu2x-converter-cpp {
+    inherit (darwin.apple_sdk.frameworks) OpenCL;
+  };
 
   wakatime = pythonPackages.callPackage ../tools/misc/wakatime { };