Merge pull request #218 from thecodeshop/cmake-xcode

enhance CMake support for Xcode native builds

CMakeLists.txt

@@ -33,12 +33,13 @@ 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' "
-          "subdirectory and run CMake from there.")
+          "Makefiles distributed with mruby. Delete any created 'CMakeFiles' "
+          "subdirectory and 'CMakeCache.txt' file from the current directory, "
+          "change to the 'build' subdirectory, and re-run CMake from there.")
 endif()
 
 if(COMMAND cmake_policy)