[packagechooser] Add CMake knobs to enable/disable item choices

- AppData and AppStream can be disabled independently of finding
   their requirements (possibly useful if you want to ignore
   AppStream even when it's installed in your build environment).
 - Add a little top-level documentation about WITH_

CMakeLists.txt

@@ -25,6 +25,9 @@
 #   SKIP_MODULES    : a space or semicolon-separated list of directory names
 #                     under src/modules that should not be built.
 #   USE_<foo>       : fills in SKIP_MODULES for modules called <foo>-<something>
+#   WITH_<foo>      : try to enable <foo> (these usually default to ON). For
+#                     a list of WITH_<foo> grep CMakeCache.txt after running
+#                     CMake once.
 #   BUILD_<foo>     : choose additional things to build
 #   DEBUG_<foo>     : special developer flags for debugging
 #

src/modules/packagechooser/CMakeLists.txt

@@ -5,13 +5,21 @@ set( _extra_src "" )
 ### OPTIONAL AppData XML support in PackageModel
 #
 #
+option( WITH_APPDATA "Support appdata: items in PackageChooser (requires QtXml)" ON )
+if ( WITH_APPDATA )
     find_package(Qt5 COMPONENTS Xml)
     if ( Qt5Xml_FOUND )
         add_definitions( -DHAVE_XML )
         list( APPEND _extra_libraries Qt5::Xml )
         list( APPEND _extra_src ItemAppData.cpp )
     endif()
+endif()
 
+### OPTIONAL AppStream support in PackageModel
+#
+#
+option( WITH_APPSTREAM "Support appstream: items in PackageChooser (requires libappstream-qt)" ON )
+if ( WITH_APPSTREAM )
     find_package(AppStreamQt)
     set_package_properties(
         AppStreamQt PROPERTIES
@@ -25,6 +33,7 @@ if ( AppStreamQt_FOUND )
         list( APPEND _extra_libraries AppStreamQt )
         list( APPEND _extra_src ItemAppStream.cpp )
     endif()
+endif()
 
 calamares_add_plugin( packagechooser
     TYPE viewmodule