Fix build with Xcode 10

For some reason, Mac client is the only project that references
libtransmission files as if they were a part of its own project, and
inconsistently so. Make the include/import style uniform an in line with
other projects.

Adjust Mac client include directory settings a bit along the way.

macosx/PrefsController.h

@@ -21,7 +21,8 @@
  *****************************************************************************/
 
 #import <Cocoa/Cocoa.h>
-#import <transmission.h>
+
+#include <libtransmission/transmission.h>
 
 @class PortChecker;