Fix the mac/unix/linux detection logic

tests/Tests/SKTest.cs

@@ -18,10 +18,37 @@ namespace SkiaSharp.Tests
 		protected static bool IsUnix => IsLinux || IsMac;
 		protected static bool IsWindows => RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
 #else
+		private 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;
+			}
+		}
+
 		protected const string PathToFonts = "fonts";
 		protected const string PathToImages = "images";
 
-		protected static bool IsMac => Environment.OSVersion.Platform == PlatformID.MacOSX;
+		protected static bool IsMac => MacPlatformDetector.IsMac.Value;
 		protected static bool IsUnix => Environment.OSVersion.Platform == PlatformID.Unix || IsMac;
 		protected static bool IsLinux => IsUnix && !IsMac;
 		protected static bool IsWindows => !IsUnix;