@LCFraser is in the middle of implementing 64 bit support for Mac which also resolves all or most of the reasons we can't currently build against anything higher than 10.9. Here's the relevant PR to your error https://github.com/livecode/livecode/pull/3167
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Unfortunately the MacOSX 10.11 SDK has removed quite a lot of APIs that the 32-bit version of LiveCode relies on. I'm actually installing Xcode 6.4 at the moment so that I can get LiveCode to build on OS X 10.11!
I know that Fraser currently plans to update all those bits as part of the 64-bit Mac support work.
LiveCode Open Source Team — @PeterTBBrett — peter.brett@livecode.com