CMake: support out-of-tree builds

- The variables that are set for out-of-tree builds are prefixed
  with  to avoid name clashes; make the module-infrastructure
  respect those instead of the in-tree variable names.
- .. and then duplicate the in-tree variables to the out-of-tree
  variables, so we only need one set of module instructions.

CMakeLists.txt

@@ -359,6 +359,13 @@ if( NOT PYTHONLIBS_FOUND OR NOT PYTHONQT_FOUND )
     set( WITH_PYTHONQT OFF )
 endif()
 
+# Now we know the state of the ABI-options, copy them into "Calamares_"
+# prefixed variables, to match how the variables would-be-named
+# when building out-of-tree.
+set(Calamares_WITH_PYTHON ${WITH_PYTHON})
+set(Calamares_WITH_PYTHONQT ${WITH_PYTHONQT})
+set(Calamares_WITH_QML ${WITH_QML})
+
 ### Transifex Translation status
 #
 # Construct language lists for use. If there are p_tx* variables,

CMakeModules/CalamaresAddModuleSubdirectory.cmake

@@ -70,11 +70,11 @@ function( calamares_add_module_subdirectory )
         # _mod_testing boolean if the module should be added to the loadmodule tests
         file(STRINGS "${_mod_dir}/module.desc" MODULE_INTERFACE REGEX "^interface")
         if ( MODULE_INTERFACE MATCHES "pythonqt" )
-            set( _mod_enabled ${WITH_PYTHONQT} )
+            set( _mod_enabled ${Calamares_WITH_PYTHONQT} )
             set( _mod_reason "No PythonQt support" )
             set( _mod_testing OFF )
         elseif ( MODULE_INTERFACE MATCHES "python" )
-            set( _mod_enabled ${WITH_PYTHON} )
+            set( _mod_enabled ${Calamares_WITH_PYTHON} )
             set( _mod_reason "No Python support" )
             set( _mod_testing ON )  # Will check syntax and imports, at least
         elseif ( MODULE_INTERFACE MATCHES "qtplugin" )