CMake: Port the 'ltcg' feature

This maps to the CMake variable CMAKE_INTERPROCEDURAL_OPTIMIZATION.

Change-Id: Id0ce48f176b95c27e74ab80276e89503b1660f79
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

configure.cmake

@@ -509,6 +509,12 @@ qt_feature("precompile_header"
     CONDITION BUILD_WITH_PCH
 )
 qt_feature_config("precompile_header" QMAKE_PRIVATE_CONFIG)
+qt_feature("ltcg"
+    LABEL "Using LTCG"
+    AUTODETECT 1
+    CONDITION CMAKE_INTERPROCEDURAL_OPTIMIZATION
+)
+qt_feature_config("ltcg" QMAKE_PRIVATE_CONFIG)
 qt_feature("reduce_exports" PRIVATE
     LABEL "Reduce amount of exported symbols"
     CONDITION NOT WIN32 AND CMAKE_CXX_COMPILE_OPTIONS_VISIBILITY

util/cmake/configurejson2cmake.py

@@ -852,7 +852,10 @@ def get_feature_mapping():
             "condition": "NOT QT_FEATURE_icu AND QT_FEATURE_textcodec AND ( TEST_posix_iconv OR TEST_sun_iconv )"
         },
         "incredibuild_xge": None,
-        "ltcg": None,
+        "ltcg": {
+            "autoDetect": "1",
+            "condition": "CMAKE_INTERPROCEDURAL_OPTIMIZATION"
+        },
         "msvc_mp": None,
         "optimize_debug": None,
         "optimize_size": None,