[packagechooser] Fix fallthrough situation

- Add a FALLTHRU macro to annotate fallthrough situations in both
   Clang and GCC,
 - Annotate intentional fallthroughs.
 - Add missing break which meant that the selection mode was
   always multiple-selection.

CMakeLists.txt

@@ -198,7 +198,7 @@ if( CMAKE_CXX_COMPILER_ID MATCHES "Clang" )
     )
         string( APPEND CMAKE_CXX_FLAGS " ${CLANG_WARNINGS}" )
     endforeach()
-    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='//'" )
+    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='//' -DFALLTHRU='[[clang::fallthrough]]'")
 
     # Third-party code where we don't care so much about compiler warnings
     # (because it's uncomfortable to patch) get different flags; use
@@ -225,7 +225,7 @@ else()
     set( SUPPRESS_3RDPARTY_WARNINGS "" )
     set( SUPPRESS_BOOST_WARNINGS "" )
 
-    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='__builtin_unreachable();'" )
+    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='__builtin_unreachable();' -DFALLTHRU='/* */'" )
 endif()
 
 # Use mark_thirdparty_code() to reduce warnings from the compiler

src/modules/packagechooser/PackageChooserPage.cpp

@@ -41,9 +41,12 @@ PackageChooserPage::PackageChooserPage( PackageChooserMode mode, QWidget* parent
     switch ( mode )
     {
     case PackageChooserMode::Optional:
+        FALLTHRU;
     case PackageChooserMode::Required:
         ui->products->setSelectionMode( QAbstractItemView::SingleSelection );
+        break;
     case PackageChooserMode::OptionalMultiple:
+        FALLTHRU;
     case PackageChooserMode::RequiredMultiple:
         ui->products->setSelectionMode( QAbstractItemView::ExtendedSelection );
     }