fix build without QwtPolar

Author: jef-n

src/app/CMakeLists.txt

@@ -375,6 +375,9 @@ IF (WITH_QWTPOLAR)
 
     SET(QWTPOLAR_LIBRARY "")
   ENDIF(WITH_INTERNAL_QWTPOLAR)
+ELSE(WITH_QWTPOLAR)
+  SET(QWTPOLAR_LIBRARY "")
+  SET(QWTPOLAR_INCLUDE_DIR "")
 ENDIF(WITH_QWTPOLAR)
 
 IF (POSTGRES_FOUND)