Features: Add cxx_extern_templates.
Fix inheriting constructors version.
Features: Add cxx_alignas.

cxx_alignof will be a separate feature known to CMake, because
both can be implemented with separate backward compatibility
versions.
Features: Add cxx_alignof.
Features: Add cxx_attributes.
Merge topic 'cxx11-features' into next

647922af Features: Add cxx_attributes.
c4879c4f Features: Add cxx_alignof.
16e781e2 Features: Add cxx_alignas.
2653f1ca Fix inheriting constructors version.
41580820 Features: Add cxx_extern_templates.