Xcode: Use sysroot and deployment target to identify compiler

Use CMAKE_OSX_SYSROOT and CMAKE_OSX_DEPLOYMENT_TARGET to set the Xcode
SDKROOT and MACOSX_DEPLOYMENT_TARGET build settings.  This is necessary
because some versions of Xcode select a different compiler based on
these settings.  We need to make sure the compiler identified during
language initialization matches what will be used for the actual build.

Modules/CMakeDetermineCompilerId.cmake

@@ -215,6 +215,17 @@ Id flags: ${testflags}
     else()
       set(id_toolset "")
     endif()
+    if(CMAKE_OSX_DEPLOYMENT_TARGET)
+      set(id_deployment_target
+        "MACOSX_DEPLOYMENT_TARGET = \"${CMAKE_OSX_DEPLOYMENT_TARGET}\";")
+    else()
+      set(id_deployment_target "")
+    endif()
+    if(CMAKE_OSX_SYSROOT)
+      set(id_sdkroot "SDKROOT = \"${CMAKE_OSX_SYSROOT}\";")
+    else()
+      set(id_sdkroot "")
+    endif()
     if(NOT ${XCODE_VERSION} VERSION_LESS 3)
       set(v 3)
       set(ext xcodeproj)

Modules/CompilerId/Xcode-3.pbxproj.in

@@ -84,6 +84,8 @@
 				CONFIGURATION_BUILD_DIR = "$(BUILD_DIR)";
 				SYMROOT = .;
 				@id_toolset@
+				@id_deployment_target@
+				@id_sdkroot@
 			};
 			name = Debug;
 		};