Reduce warnings from moc-generated code

- The auto-generated code produces a lot of warnings from
   Clang 8; this obscures the more meaningful warnings from
   actual Calamares code, so tone the warnings down.
 - For Clang, set CALAMARES_MOC_OPTIONS.
 - Add convenience CMake function for automoccing. It applies
   the options as needed to a given target.

src/libcalamares/CMakeLists.txt

@@ -84,10 +84,10 @@ endif()
 add_library( calamares SHARED ${libSources} ${kdsagSources} ${utilsSources} )
 set_target_properties( calamares
     PROPERTIES
-        AUTOMOC TRUE
         VERSION ${CALAMARES_VERSION_SHORT}
         SOVERSION ${CALAMARES_VERSION_SHORT}
 )
+calamares_automoc( calamares )
 
 target_link_libraries( calamares
     LINK_PRIVATE
@@ -114,7 +114,7 @@ if ( ECM_FOUND AND BUILD_TESTING )
             Qt5::Core
             Qt5::Test
     )
-    set_target_properties( libcalamarestest PROPERTIES AUTOMOC TRUE )
+    calamares_automoc( libcalamarestest )
 endif()
 
 # Make symlink lib/calamares/libcalamares.so to lib/libcalamares.so.VERSION so

src/libcalamares/utils/moc-warnings.h

@@ -0,0 +1,4 @@
+#ifdef __clang__
+#pragma clang diagnostic ignored "-Wextra-semi-stmt"
+#pragma clang diagnostic ignored "-Wredundant-parens"
+#endif