Warn if building flatpak is enabled but some dependencies aren't found

BUG: 388295

CMakeLists.txt

@@ -64,5 +64,10 @@ set_package_properties(AppStreamQt PROPERTIES
     URL "http://www.freedesktop.org"
     PURPOSE "Required to build the PackageKit backend"
     TYPE OPTIONAL)
+set_package_properties(FLATPAK PROPERTIES
+    DESCRIPTION "Library that exposes flatpak repositories"
+    URL "http://www.freedesktop.org"
+    PURPOSE "Required to build the Flatpak backend"
+    TYPE OPTIONAL)
 
 feature_summary(WHAT ALL FATAL_ON_MISSING_REQUIRED_PACKAGES)

libdiscover/backends/CMakeLists.txt

@@ -24,6 +24,8 @@ endif()
 option(BUILD_FlatpakBackend "Build Flatpak support" "ON")
 if(FLATPAK_FOUND AND AppStreamQt_FOUND AND BUILD_FlatpakBackend)
     add_subdirectory(FlatpakBackend)
+elseif(BUILD_FlatpakBackend)
+    message(WARNING "BUILD_FlatpakBackend enabled but Flatpak=${FLATPAK_FOUND} or AppStreamQt=${AppStreamQt_FOUND} not found")
 endif()
 
 option(BUILD_SnapBackend "Build Snap support. Still a proof of concept" "OFF")