Xcode: Properly mark SYSTEM includes as such

We now populate the per-language flags in addition to the header
search paths stored in HEADER_SEARCH_PATHS. This preserves include
paths for GNU assembly files (cmake/cmake#16449) and also provides
SYSTEM include semantics.

Closes: cmake/cmake#15687

Source/cmGlobalXCodeGenerator.cxx

@@ -1977,6 +1977,22 @@ void cmGlobalXCodeGenerator::CreateBuildSettings(cmGeneratorTarget* gtgt,
     buildSettings->AddAttribute("HEADER_SEARCH_PATHS", dirs.CreateList());
   }
 
+  if (this->XcodeVersion >= 60) {
+    // Add those per-language flags in addition to HEADER_SEARCH_PATHS to gain
+    // system include directory awareness. We need to also keep on setting
+    // HEADER_SEARCH_PATHS to work around a missing compile options flag for
+    // GNU assembly files (#16449)
+    for (std::set<std::string>::iterator li = languages.begin();
+         li != languages.end(); ++li) {
+      std::string includeFlags = this->CurrentLocalGenerator->GetIncludeFlags(
+        includes, gtgt, *li, true, false, configName);
+
+      if (!includeFlags.empty()) {
+        cflags[*li] += " " + includeFlags;
+      }
+    }
+  }
+
   bool same_gflags = true;
   std::map<std::string, std::string> gflags;
   std::string const* last_gflag = 0;

Tests/IncludeDirectories/CMakeLists.txt

@@ -3,7 +3,9 @@ project(IncludeDirectories)
 
 if (((CMAKE_C_COMPILER_ID STREQUAL GNU AND CMAKE_C_COMPILER_VERSION VERSION_GREATER 4.4)
     OR CMAKE_C_COMPILER_ID STREQUAL Clang OR CMAKE_C_COMPILER_ID STREQUAL AppleClang)
-    AND (CMAKE_GENERATOR STREQUAL "Unix Makefiles" OR CMAKE_GENERATOR STREQUAL "Ninja"))
+    AND (CMAKE_GENERATOR STREQUAL "Unix Makefiles"
+      OR CMAKE_GENERATOR STREQUAL "Ninja"
+      OR (CMAKE_GENERATOR STREQUAL "Xcode" AND NOT XCODE_VERSION VERSION_LESS 6.0)))
   include(CheckCXXCompilerFlag)
   check_cxx_compiler_flag(-Wunused-variable run_sys_includes_test)
   if(run_sys_includes_test)

Tests/IncludeDirectories/SystemIncludeDirectories/CMakeLists.txt

@@ -15,10 +15,17 @@ target_include_directories(upstream SYSTEM PUBLIC
 )
 
 add_library(config_specific INTERFACE)
-set(testConfig ${CMAKE_BUILD_TYPE})
-target_include_directories(config_specific SYSTEM INTERFACE
-  "$<$<CONFIG:${testConfig}>:${CMAKE_CURRENT_SOURCE_DIR}/config_specific>"
-)
+if(CMAKE_GENERATOR STREQUAL "Xcode")
+  # CMAKE_BUILD_TYPE does not work here for multi-config generators
+  target_include_directories(config_specific SYSTEM INTERFACE
+    "${CMAKE_CURRENT_SOURCE_DIR}/config_specific"
+  )
+else()
+  set(testConfig ${CMAKE_BUILD_TYPE})
+  target_include_directories(config_specific SYSTEM INTERFACE
+    "$<$<CONFIG:${testConfig}>:${CMAKE_CURRENT_SOURCE_DIR}/config_specific>"
+  )
+endif()
 
 add_library(consumer consumer.cpp)
 target_link_libraries(consumer upstream config_specific)