[plasmalnf] Make the module optional

- Check for presence of KDE Frameworks for Plasma & Package
 - Explain when module is skipped

src/modules/plasmalnf/CMakeLists.txt

@@ -1,6 +1,22 @@
 find_package(ECM ${ECM_VERSION} REQUIRED NO_MODULE)
 
-find_package( KF5 5.29 REQUIRED CoreAddons Plasma Package )
+# Requires a sufficiently recent Plasma framework, but also
+# needs a runtime support component (which we don't test for).
+set( lnf_ver 5.41 )
+
+find_package( KF5Plasma ${lnf_ver} )
+find_package( KF5Package ${lnf_ver} )
+set_package_properties(
+    KF5Plasma PROPERTIES
+    PURPOSE "For Plasma Look-and-Feel selection"
+)
+set_package_properties(
+    KF5Package PROPERTIES
+    PURPOSE "For Plasma Look-and-Feel selection"
+)
+
+if ( KF5Plasma_FOUND AND KF5Package_FOUND )
+    find_package( KF5 ${lnf_ver} REQUIRED CoreAddons Plasma Package )
 
     calamares_add_plugin( plasmalnf
         TYPE viewmodule
@@ -20,3 +36,6 @@ calamares_add_plugin( plasmalnf
             KF5::Plasma
         SHARED_LIB
     )
+else()
+    calamares_skip_module( "plasmalnf (missing requirements)" )
+endif()