Fix bad path manipulation in macOS VM

We should have been using `stringByAppendingPathComponent:`, not
`stringByAppendingPathExtension`. This didn't matter until Sierra, where
they improved/increased error checking.

@bpollack #39

platforms/iOS/vm/OSX/sqMacUnixExternalPrims.m

@@ -111,7 +111,7 @@
 	NSString    *libName;
 
 	libName = [dirNameString stringByAppendingPathComponent: @(moduleName)];
-	libName = [libName stringByAppendingPathExtension: @"bundle/Contents/MacOS/"];
+	libName = [libName stringByAppendingPathComponent: @"bundle/Contents/MacOS/"];
 	libName = [libName stringByAppendingPathComponent: @(moduleName)];
 	handle = tryLoadingInternals(libName);
 	if (handle)