CMake: Make LTO build work

We don't need to check FEATURE_ltcg, just add -fno-lto unconditionally.
That makes QtCore compile with -DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON.

Change-Id: Icb2516126f674e7b8bb3fffd16ada2c71d7334aa
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtModuleHelpers.cmake

@@ -195,7 +195,7 @@ function(qt_internal_add_module target)
         endif()
     endif()
 
-    if(FEATURE_ltcg AND GCC AND is_static_lib)
+    if((FEATURE_ltcg OR CMAKE_INTERPROCEDURAL_OPTIMIZATION) AND GCC AND is_static_lib)
         # CMake <= 3.19 appends -fno-fat-lto-objects for all library types if
         # CMAKE_INTERPROCEDURAL_OPTIMIZATION is enabled. Static libraries need
         # the opposite compiler option.

src/corelib/CMakeLists.txt

@@ -605,7 +605,7 @@ target_sources(Core PRIVATE ${core_version_tagging_files})
 
 # Disable LTO, as the symbols disappear somehow under GCC
 # (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48200)
-if(GCC AND FEATURE_ltcg)
+if(GCC)
     set_source_files_properties(${core_version_tagging_files}
                                 PROPERTIES COMPILE_OPTIONS "-fno-lto")
 endif()

src/corelib/global/qversiontagging.cpp

@@ -102,7 +102,6 @@ make_versioned_symbol(SYM, QT_VERSION_MAJOR, 14, "@");
 make_versioned_symbol(SYM, QT_VERSION_MAJOR, 15, "@");
 #endif
 #if QT_VERSION_MINOR > 16
-// We don't expect there will be a Qt 5.17
 #  error "Please update this file with more Qt versions."
 #endif