Threads backend doesn't compile with Clang on OS X

[keitat]

It will be great if I can get the latest information of Kokkos support for OS X; in particular Clang compilers. The information in README is a bit outdated.

[keitat]

=== Message from Danny Dunlavy ====
I am using Xcode 8.2:
Apple LLVM version 8.0.0 (clang-800.0.42.1)
Target: x86_64-apple-darwin15.6.0

which does not contain cstdalign.h:

/Users/dmdunla/dev/hpda-tensors/tensor/sparten/tpl/kokkos/core/src/Threads/Kokkos_ThreadsExec.hpp:53:10: fatal error: 'cstdalign' file not found
#include

I looked online and it is not clear whether Xcode 9 has it either. So this dependency means that Apple clang may not be an option for building.

[ibaned]

Can confirm this is still an issue with:

Apple LLVM version 9.0.0 (clang-900.0.38)

[ibaned]

It is not an issue with:

clang version 5.0.0-svn301099-1~exp1

on Ubuntu.
However, on both the OS X and Ubuntu platforms I have, Kokkos Core and its tests still compile with <cstdalign> removed, under Clang, suggesting the header may not be needed.

[ibaned]

The include was added in ead19d5 and only has to do with alignas, but grep says that kokkos/core/ contains no references to alignas, we instead use __attribute__((aligned(N))) for alignment control. This further suggests it can be removed.

[ibaned]

Pull request #1242 should fix this

[ibaned]

@keitat in short, Kokkos needs to be able to build with the latest Clang on OS X, it doesn't, and we're fixing that.