Accept 'tools/buildinfo' as an alternative path to 'buildinfo' due to…

… an nuget bug. (#70)

nuget doesn't like files that start with 'build*' in the root nuget directory
[1], so avoid it by putting 'buildinfo' in 'tools/buildinfo' instead.

[1]: NuGet/Home#8810

Xamarin.MacDev/MonoTouchSdk.cs

@@ -285,8 +285,13 @@ public static bool ValidateSdkLocation (string sdkDir, out bool hasUsrSubdir)
 		ExtendedVersion extended_version;
 		public ExtendedVersion ExtendedVersion {
 			get {
-				if (extended_version == null)
+				if (extended_version == null) {
 					extended_version = ExtendedVersion.Read (Path.Combine (SdkDir, "buildinfo"));
+					if (extended_version == null) {
+						// 'buildinfo' doesn't work in a nuget package because of https://github.com/NuGet/Home/issues/8810, so use 'tools/buildinfo' instead.
+						extended_version = ExtendedVersion.Read (Path.Combine (SdkDir, "tools", "buildinfo"));
+					}
+				}
 				return extended_version;
 			}
 		}