[CMake] Xcode generator results in a project file without optimization

[Siyuan Ren]

I have CMake 3.5.2.

I write a simple CMakeLists.txt like the below

```
project(mytest)
add_executable(mytest test.cpp)
```

Then generate the Xcode project file with `cmake -G Xcode .`. Open the
project file in Xcode, and I found the optimization level on all kinds
of build, including "Debug", "Release", "RelWithDebInfo",
"MinSizeRel", to be "None (-O0)".

Do I do something wrong? Or CMake Xcode generator cannot even handle
the simplest project right?