find_package: only depend on Qt if BUILD_QT_UI is set to ON (#223)

Author: TheOneRing

cmake/AppImageUpdateConfig.cmake.in

@@ -3,6 +3,10 @@
 # look up dependencies
 find_package(zsync2 REQUIRED)
 find_package(libappimage REQUIRED)
-find_package(Qt5 REQUIRED COMPONENTS Core Widgets)
+
+# BUILD_QT_UI
+if(@BUILD_QT_UI@)
+    find_package(Qt5 REQUIRED COMPONENTS Core Widgets)
+endif()
 
 include("${CMAKE_CURRENT_LIST_DIR}/AppImageUpdateTargets.cmake")