Assume not having libc means this is not macOS

We know that libc will always be present on macOS, so if it is not
there, we can assume this is not macOS.

More generally, use a try/finally to avoid leaking the allocated buffer
when libc is not found and set `checkOS` properly so we don't try to
check again.

This corrects Unity case 1038918, and in general helps our platform
story int he class libraries a little bit, until we get better platform
detection support at runtime.

Author: joshpeterson

mcs/class/System/System/Platform.cs

@@ -59,26 +59,40 @@ private static void CheckOS() {
 			}
 
 			IntPtr buf = Marshal.AllocHGlobal (8192);
-			if (uname (buf) == 0) {
-				string os = Marshal.PtrToStringAnsi (buf);
-				switch (os) {
-				case "Darwin":
-					isMacOS = true;
-					break;
-				case "FreeBSD":
-					isFreeBSD = true;
-					break;
+			try {
+				if (uname (buf) == 0) {
+					string os = Marshal.PtrToStringAnsi (buf);
+					switch (os) {
+					case "Darwin":
+						isMacOS = true;
+						break;
+					case "FreeBSD":
+						isFreeBSD = true;
+						break;
+					}
 				}
 			}
-			Marshal.FreeHGlobal (buf);
-			checkedOS = true;
+			finally {
+				Marshal.FreeHGlobal (buf);
+				checkedOS = true;
+			}
 		}
 #endif
 
 		public static bool IsMacOS {
 			get {
 				if (!checkedOS)
+#if UNITY
+					try {
+						CheckOS();
+					}
+					catch (DllNotFoundException e) {
+						// libc does not exist, so this is not MacOS
+						isMacOS = false;
+					}
+#else
 					CheckOS();
+#endif
 				return isMacOS;
 			}
 		}