boost-build: fix darwin build

pkgs/development/tools/boost-build/darwin-default-toolset.patch

···
+diff --git a/src/build-system.jam b/src/build-system.jam
+index 60425c54..c6842217 100644
+--- a/src/build-system.jam
++++ b/src/build-system.jam
+@@ -644,7 +644,7 @@ local rule should-clean-project ( project )
+             }
+             else if [ os.name ] = MACOSX
+             {
+-                default-toolset = darwin ;
++                default-toolset = clang-darwin ;
+             }
+         }

pkgs/development/tools/boost-build/default.nix

···
     sha256 = "1r4rwlq87ydmsdqrik4ly5iai796qalvw7603mridg2nwcbbnf54";
   };
 
+  patches = [
+    # Upstream defaults to gcc on darwin, but we use clang.
+    ./darwin-default-toolset.patch
+  ];
+
   nativeBuildInputs = [
     bison
   ];