cxx features: add check for override

Source/Checks/cm_cxx_features.cmake

@@ -32,6 +32,7 @@ function(cm_check_cxx_feature name)
 endfunction()
 
 if(CMAKE_CXX_STANDARD)
+  cm_check_cxx_feature(override)
   cm_check_cxx_feature(unordered_map)
   cm_check_cxx_feature(unordered_set)
 endif()

Source/Checks/cm_cxx_override.cxx

+struct Foo
+{
+  virtual int test() const = 0;
+};
+
+struct Bar : Foo
+{
+  int test() const override { return 0; }
+};
+
+int test(Foo const& foo)
+{
+  return foo.test();
+}
+
+int main()
+{
+  Bar const bar;
+  return test(bar);
+}

Source/cmConfigure.cmake.h.in

@@ -29,9 +29,16 @@
 #cmakedefine CMAKE_USE_ELF_PARSER
 #cmakedefine CMAKE_USE_MACH_PARSER
 #cmakedefine CMAKE_ENCODING_UTF8
+#cmakedefine CMake_HAVE_CXX_OVERRIDE
 #cmakedefine CMake_HAVE_CXX_UNORDERED_MAP
 #cmakedefine CMake_HAVE_CXX_UNORDERED_SET
 #define CMAKE_BIN_DIR "/@CMAKE_BIN_DIR@"
 #define CMAKE_DATA_DIR "/@CMAKE_DATA_DIR@"
 
+#ifdef CMake_HAVE_CXX_OVERRIDE
+#define CM_OVERRIDE override
+#else
+#define CM_OVERRIDE
+#endif
+
 #endif

bootstrap

@@ -1272,6 +1272,7 @@ cmake_report cmConfigure.h${_tmp} "#define CMAKE_BOOTSTRAP_BINARY_DIR \"${CMAKE_
 cmake_report cmConfigure.h${_tmp} "#define CMAKE_BIN_DIR \"/bootstrap-not-insalled\""
 cmake_report cmConfigure.h${_tmp} "#define CMAKE_DATA_DIR \"/bootstrap-not-insalled\""
 cmake_report cmConfigure.h${_tmp} "#define CMAKE_BOOTSTRAP"
+cmake_report cmConfigure.h${_tmp} "#define CM_OVERRIDE"
 
 # Regenerate configured headers
 for h in Configure VersionConfig; do