Corrected the Mac detection logic

cake/UtilsManaged.cake

@@ -1,3 +1,4 @@
+using System.Runtime.InteropServices;
 
 var RunNuGetRestore = new Action<FilePath> ((solution) =>
 {
@@ -82,8 +83,35 @@ var ClearSkiaSharpNuGetCache = new Action (() => {
     }
 });
 
+internal static class MacPlatformDetector
+{
+    internal static readonly Lazy<bool> IsMac = new Lazy<bool> (IsRunningOnMac);
+
+    [DllImport ("libc")]
+    static extern int uname (IntPtr buf);
+
+    static bool IsRunningOnMac ()
+    {
+        IntPtr buf = IntPtr.Zero;
+        try {
+            buf = Marshal.AllocHGlobal (8192);
+            // This is a hacktastic way of getting sysname from uname ()
+            if (uname (buf) == 0) {
+                string os = Marshal.PtrToStringAnsi (buf);
+                if (os == "Darwin")
+                    return true;
+            }
+        } catch {
+        } finally {
+            if (buf != IntPtr.Zero)
+                Marshal.FreeHGlobal (buf);
+        }
+        return false;
+    }
+}
+
 var IsRunningOnMac = new Func<bool> (() => {
-    return System.Environment.OSVersion.Platform == PlatformID.MacOSX;
+    return System.Environment.OSVersion.Platform == PlatformID.MacOSX || MacPlatformDetector.IsMac.Value;
 });
 
 var IsRunningOnLinux = new Func<bool> (() => {