treewide: Use *Platform.extensions

pkgs/test/stdenv-inputs/default.nix

@@ -1,8 +1,6 @@
 { stdenv }:
 
 let
-  shlib = if stdenv.isDarwin then "dylib" else "so";
-
   foo = stdenv.mkDerivation {
     name = "foo-test";
 
@@ -13,7 +11,10 @@ let
       $CC -o $out/bin/foo ${./cc-main.c}
       chmod +x $out/bin/foo
       cp ${./foo.c} $out/include/foo.h
-      $CC -shared ${stdenv.lib.optionalString stdenv.isDarwin "-Wl,-install_name,$out/lib/libfoo.dylib"} -o $out/lib/libfoo.${shlib} ${./foo.c}
+      $CC -shared \
+        ${stdenv.lib.optionalString stdenv.isDarwin "-Wl,-install_name,$out/lib/libfoo.dylib"} \
+        -o $out/lib/libfoo${stdenv.hostPlatform.extensions.sharedLibrary} \
+        ${./foo.c}
     '';
   };
 
@@ -28,7 +29,10 @@ let
       $CC -o $out/bin/bar ${./cc-main.c}
       chmod +x $out/bin/bar
       cp ${./bar.c} $dev/include/bar.h
-      $CC -shared ${stdenv.lib.optionalString stdenv.isDarwin "-Wl,-install_name,$dev/lib/libbar.dylib"} -o $dev/lib/libbar.${shlib} ${./bar.c}
+      $CC -shared \
+        ${stdenv.lib.optionalString stdenv.isDarwin "-Wl,-install_name,$dev/lib/libbar.dylib"} \
+        -o $dev/lib/libbar${stdenv.hostPlatform.extensions.sharedLibrary} \
+        ${./bar.c}
     '';
   };
 in