What versions of OS X, Xcode, iOS SDK to build externals?

[Robert L.]

I’ve posted this to the forums as well, but I know that the mailing list 
has a slightly different audience.  

I’m trying my hand at building some simple externals, but I can't even 
seem to make one attempt to build. Xcode is telling me that my project has 
"0 targets, missing base SDK". I think it may be due to using too recent 
version of Xcode (and thus missing older iOS SDKs).

Currently I am on OS X Yosemite 10.10.1, Xcode 6.1.1 (which lets me target 
iOS 6.0 to 8.1), with the LiveCode iOS externals R14 SDK Support. 

Are there any tricks to build an external with the most recent tools? If 
not, what is the most recent combination of OS X and Xcode that is known 
to work for building iOS externals? I can build a virtual machine if 
needed.

Any advice in getting started building externals would be greatly 
appreciated.

Thanks, 

Robert