LLVM 3.8.1 fails on armel on Debian

Hello

I am trying to build LLVM on armel but it fails on:

/«PKGBUILDDIR»/include/llvm/Support/ThreadPool.h: In member function 'std::shared_future<void> llvm::ThreadPool::async(Function&&, Args&& ...)':
/«PKGBUILDDIR»/include/llvm/Support/ThreadPool.h:78:77: error: return type 'class std::shared_future<void>' is incomplete
    inline std::shared_future<VoidTy> async(Function &&F, Args &&... ArgList) {
                                                                              ^
/«PKGBUILDDIR»/include/llvm/Support/ThreadPool.h: In member function 'std::shared_future<void> llvm::ThreadPool::async(Function&&)':
/«PKGBUILDDIR»/include/llvm/Support/ThreadPool.h:96:57: error: return type 'class std::shared_future<void>' is incomplete
    inline std::shared_future<VoidTy> async(Function &&F) {
                                                          ^
/«PKGBUILDDIR»/lib/Support/ThreadPool.cpp: In lambda function:
/«PKGBUILDDIR»/lib/Support/ThreadPool.cpp:34:24: error: aggregate 'llvm::ThreadPool::PackagedTaskTy Task' has incomplete type and cannot be defined
          PackagedTaskTy Task;
full log:
https://buildd.debian.org/status/fetch.php?pkg=llvm-toolchain-3.8&arch=armel&ver=1%3A3.8.1-5&stamp=1469659795
This will probably affect 3.9 too.

Asking to Renato some help, he mentioned:
http://lists.llvm.org/pipermail/llvm-dev/2016-May/099761.html

But adding "-DCOMPILER_RT_BUILD_BUILTINS=OFF" to CMake didn't fix my issue...

Any other ideas?

Thanks,
Sylvestre