CMake: Rewrite of VampirTrace without wrapper

Rewrite of the VampirTrace Support in CMake and close of #269.

We avoid using the `vtc*` wrappers by using the [5.11.2+](http://tu-dresden.de/die_tu_dresden/zentrale_einrichtungen/zih/forschung/projekte/vampirtrace/dateien/VT-ChangeLog-5.14.4.txt) options `-vt:showme-<compile|link>` instead.

Support is (still) gcc only I guess.

**Note:** *(hypnos specific)* ~~compile on a `k20` node since vampirtrace needs driver functions from `-lcuda`. Use *cuda 5.0* since the vt module was build with that!~~ does not work... may compile your own vampirtrace -.-

***

This change checks if `VT_ROOT` was set by the user.

~~One should **not** change the compiler in cmake itself since this causes problems during cross compiles, e.g. on titan.~~
~~Better set this stuff before calling cmake with:~~
```bash
export CC="`which vtcc` -target=compute_node"
export CXX="`which vtcxx` -target=compute_node"
```
(note: the `-target` flags are titan specific).