CMake: prefer normal C++17 [[fallthrough]] annotation

CMakeLists.txt

@@ -228,7 +228,7 @@ if( CMAKE_CXX_COMPILER_ID MATCHES "Clang" )
     )
         string( APPEND CMAKE_CXX_FLAGS " ${CLANG_WARNINGS}" )
     endforeach()
-    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='//' -DFALLTHRU='[[clang::fallthrough]]'")
+    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='//'")
 
     # Third-party code where we don't care so much about compiler warnings
     # (because it's uncomfortable to patch) get different flags; use
@@ -247,7 +247,7 @@ else()
 
     set( SUPPRESS_3RDPARTY_WARNINGS "" )
 
-    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='__builtin_unreachable();' -DFALLTHRU='/* */'" )
+    set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNOTREACHED='__builtin_unreachable();'" )
 endif()
 
 # Use mark_thirdparty_code() to reduce warnings from the compiler

src/libcalamaresui/viewpages/ExecutionViewStep.cpp

@@ -42,7 +42,7 @@ makeSlideshow( QWidget* parent )
         return new Calamares::SlideshowPictures( parent );
 #ifdef WITH_QML
     case 1:
-        FALLTHRU;
+        [[fallthrough]];
     case 2:
         return new Calamares::SlideshowQML( parent );
 #endif

src/modules/packagechooser/PackageChooserPage.cpp

@@ -33,12 +33,12 @@ PackageChooserPage::PackageChooserPage( PackageChooserMode mode, QWidget* parent
     switch ( mode )
     {
     case PackageChooserMode::Optional:
-        FALLTHRU;
+        [[fallthrough]];
     case PackageChooserMode::Required:
         ui->products->setSelectionMode( QAbstractItemView::SingleSelection );
         break;
     case PackageChooserMode::OptionalMultiple:
-        FALLTHRU;
+        [[fallthrough]];
     case PackageChooserMode::RequiredMultiple:
         ui->products->setSelectionMode( QAbstractItemView::ExtendedSelection );
     }

src/modules/tracking/Config.cpp

@@ -182,10 +182,10 @@ enableLevelsBelow( Config* config, TrackingType level )
     {
     case TrackingType::UserTracking:
         config->userTracking()->setTracking( TrackingStyleConfig::TrackingState::EnabledByUser );
-        FALLTHRU;
+        [[fallthrough]];
     case TrackingType::MachineTracking:
         config->machineTracking()->setTracking( TrackingStyleConfig::TrackingState::EnabledByUser );
-        FALLTHRU;
+        [[fallthrough]];
     case TrackingType::InstallTracking:
         config->installTracking()->setTracking( TrackingStyleConfig::TrackingState::EnabledByUser );
         break;