Audiff test failures for Clang/MacOS

[jzmaddock]

In order to get us green again, I've disabled 2 tests in test_autodiff_8.cpp for Clang/MacOS:

math/test/test_autodiff_8.cpp

Line 12 in a243640

#if !defined(__clang__) || !defined(__APPLE__) || !defined(__MACH__)

and

math/test/test_autodiff_8.cpp

Line 135 in a243640

#if !defined(__clang__) || !defined(__APPLE__) || !defined(__MACH__)

One for @pulver.