Update CMake to enable normal Xcode workflows

Similar to the existing CMake Visual Studio IDE support, allow CMake to
generate Xcode project files in mruby's root directory. This enables
workflows normally expected by Xcode IDE users.

CMakeLists.txt

@@ -33,8 +33,8 @@ project(mruby C)
 # TODO stop polluting source tree with CMakeFiles/ and CMakeCache.txt
 #      on build location check failure
 # Make sure we are not trying to generate in in-tree build unless building
-# with a MSVC IDE where it's OK.
-if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR AND NOT MSVC_IDE)
+# with an MSVC or Xcode IDE where it's OK.
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR AND NOT (MSVC_IDE OR XCODE))
   message(FATAL_ERROR
           "\nIn-source builds are not allowed as CMake would overwrite the "
           "Makefiles distributed with mruby. Please change to the 'build' "