[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

@@ -5,25 +5,34 @@ set( _extra_src "" )
 ### OPTIONAL AppData XML support in PackageModel
 #
 #
-find_package(Qt5 COMPONENTS Xml)
-if ( Qt5Xml_FOUND )
-    add_definitions( -DHAVE_XML )
-    list( APPEND _extra_libraries Qt5::Xml )
-    list( APPEND _extra_src ItemAppData.cpp )
+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()
 
-find_package(AppStreamQt)
-set_package_properties(
-    AppStreamQt PROPERTIES
-    DESCRIPTION "Support for AppStream (cache) data"
-    URL "https://github.com/ximion/appstream"
-    PURPOSE "AppStream provides package data"
-    TYPE OPTIONAL
-)
-if ( AppStreamQt_FOUND )
-    add_definitions( -DHAVE_APPSTREAM )
-    list( APPEND _extra_libraries AppStreamQt )
-    list( APPEND _extra_src ItemAppStream.cpp )
+### 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
+        DESCRIPTION "Support for AppStream (cache) data"
+        URL "https://github.com/ximion/appstream"
+        PURPOSE "AppStream provides package data"
+        TYPE OPTIONAL
+    )
+    if ( AppStreamQt_FOUND )
+        add_definitions( -DHAVE_APPSTREAM )
+        list( APPEND _extra_libraries AppStreamQt )
+        list( APPEND _extra_src ItemAppStream.cpp )
+    endif()
 endif()
 
 calamares_add_plugin( packagechooser