Avoided GCC 4.9 regression in C++14 mode

GCC 4.9.0 and 4.9.1 have a regression: don't compile parenthesized return stmt in C++14 mode.
```
movable f()
{
    movable m;
    return (m); // bug on here
}
```

This PR will fix [BP x86_64 C++11 - move - doc_move_return / gcc-4.9.1~c14](http://www.boost.org/development/tests/develop/developer/output/BP%20x86_64%20C++11-boost-bin-v2-libs-move-example-doc_move_return-test-gcc-4-9-1~c14-debug-debug-symbols-off-link-static.html), and other test failures.

For more details, see: https://gcc.gnu.org/PR63437 .