boost-build: make darwin patch work across all versions

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

@@ -1,12 +0,0 @@
-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

@@ -33,10 +33,11 @@ stdenv.mkDerivation {
     sourceRoot="$sourceRoot/tools/build"
   '';
 
-  patches = [
-    # Upstream defaults to gcc on darwin, but we use clang.
-    ./darwin-default-toolset.patch
-  ];
+  # Upstream defaults to gcc on darwin, but we use clang.
+  postPatch = ''
+    substituteInPlace src/build-system.jam \
+    --replace "default-toolset = darwin" "default-toolset = clang-darwin"
+  '';
 
   nativeBuildInputs = [
     bison