Fix thread_local

[Vladimír Štill]

Currently, the thread_local keyword is not supported. Even worse, it is silently ignored:

#include <thread>
#include <cassert>

thread_local int v;

int main()
{
    v = 42;
    std::thread t( [] {
        assert( v == 0 );
        v = 16;
        assert( v == 16 );
    } );
    assert( v == 42 );
    t.join();
}

[mornfall]

Milestone renamed

[mornfall]

Indeed, this leads to LLVM thread_local. Same thing happens with function-scoped thread_local variables. Let's aim for 4.3.