Optimization Level panel is missing in Swift Compiler - Code Generation

A project of mine totally misses the possibility of adjusting the optimization level for the Swift code and thereafter defaults to optimization for all options rendeing debugging a nightmare. This is how the relative panel looks:

How do I restore the possibility of adjusting the optimization?

P.s. If you are not able to view the image, a copy is in the corresponding post on StackOverflow at:

http://stackoverflow.com/questions/34435375/optimization-level-panel-is-missing-in-swift-compiler-code-generatio