reduces spurious re-runs by only updatin gthe builder defition
when it actually changed

- now shows which new/revomed tests trigger rebuilding
- no longer limited to building with ninja

This reverts commit b50a7d9b.
It was actually missing in dune-gdt, plus typo ridden too.

Aka: how did this ever work...

[string] rename internal::{from,to}_string

Using `from_string` and `to_string` in user code inside a
`namespace internal` failed, since our variants in
 `Dune::XT::Common::internal` had the same names as the one in
`Dune::XT::Common`, but different signatures. This should fix that issue.

-this is due to GCC bug 53431

fix warnings about double ;; in gdt

remove redundant line