Instructions for further updates:
- download new version of googletest
- use ``googletest/scripts/fuse_gtest_files.py`` to obtain only ``gtest.h`` and ``gtest-all.cc``
- (optional) manually apply clang-format (on a beefy computer) to ``gtest.h``
- suppress warnings in gtest.h by inserting (compare old version of gtest.h)
```c++
 #if defined(__clang__)
 #  pragma clang system_header
 #elif defined(__GNUC__)
 #  pragma GCC system_header
 #endif
```
- insert ( compare old version of ``gtest-all.cxx``)
```c++
```
at the beginning of ``gtest-all.cxx`` and
```c++
```
at the end.

We actually need a mix of cmake magic and config.h to work
* if alberta is not used (then the config.h define is taken)
* alberta is actually used (then the dimension has to be specified for the
  target at hand with add_dune_alberta_flags(GRIDDIM $dim $target))

The old behavior only added `-lboost_system` but did not add
`-L/path/to/boost/lib`.

The old behavior only added `-lboost_system` but did not add
`-L/path/to/boost/lib`.

also add check and recheck targets that
run DTX_TEST_PROCS many tests in parallel
and re-run only failed tests accord.