CMake: unset PNG_LIBRARIES in case PNG was not found

Author: bk138

CMakeLists.txt

@@ -212,6 +212,8 @@ else()
 endif(JPEG_FOUND)
 if(PNG_FOUND)
   set(LIBVNCSERVER_HAVE_LIBPNG 1)
+else()
+  unset(PNG_LIBRARIES) # would otherwise contain -NOTFOUND, confusing target_link_libraries()
 endif(PNG_FOUND)
 if(NOT OPENSSL_FOUND)
     unset(OPENSSL_LIBRARIES) # would otherwise contain -NOTFOUND, confusing target_link_libraries()