Add support for build setting default values introduced in Xcode 11.4

From the [release notes](https://developer.apple.com/documentation/xcode_release_notes/xcode_11_4_beta_3_release_notes):

> Build settings have a new evaluation operator, default, which you can use to specify the default value of a build setting if it evaluates to empty in the context of the evaluation. For example:
> ```
> $(SETTING:default=something)
> ```
> If $(SETTING) is empty, this expression evaluates to ‘something’. The default value may itself be an expression that contains build setting evaluations. (57402718)