Skip detecting native libc dirs on darwin

This is handled before by detecting and adding SDK path which
is a centralised point for the native libc installation on darwin.

Author: kubkon

src/Compilation.zig

@@ -3792,6 +3792,13 @@ fn detectLibCIncludeDirs(
     // If linking system libraries and targeting the native abi, default to
     // using the system libc installation.
     if (link_system_libs and is_native_abi and !target.isMinGW()) {
+        if (target.isDarwin()) {
+            // For Darwin/macOS, we are all set with getSDKPath found earlier.
+            return LibCDirs{
+                .libc_include_dir_list = &[0][]u8{},
+                .libc_installation = null,
+            };
+        }
         const libc = try arena.create(LibCInstallation);
         libc.* = try LibCInstallation.findNative(.{ .allocator = arena, .verbose = true });
         return detectLibCFromLibCInstallation(arena, target, libc);

src/libc_installation.zig

@@ -185,7 +185,9 @@ pub const LibCInstallation = struct {
     pub fn findNative(args: FindNativeOptions) FindError!LibCInstallation {
         var self: LibCInstallation = .{};
 
-        if (is_windows) {
+        if (is_darwin) {
+            @panic("Darwin is handled separately via std.zig.system.darwin module");
+        } else if (is_windows) {
             if (!build_options.have_llvm)
                 return error.WindowsSdkNotFound;
             var sdk: *ZigWindowsSDK = undefined;