Have metatargets for system test executables

Eike just approached me with a question on how he can compile all static variants of ONE given system test. I think it would be nice to hack this into add_static_variants like this:

• add a meta target that goes by the name of the basename.
• have all static variants depend on the basename metatarget

That will result in a target that triggers the building of all variants. The case of only one variant has to be excluded of course!

What do you think?