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.

see http://clang.llvm.org/extra/clang-tidy/

`add_format` now only takes one argument: a directory that is used
the base dir for recursive globbing files into clang-format
Usages of the macros are adjusted accordingly. It's still possible to
use it multiple times in a project, if one for whatever reason
does not wnat to throw it at the source root. Only one `format`
target will be generated either way, with one subtarget per
`add_format` invocation.

this closes #10

clang format discovery is improved a lot by using cmake
builtins, allowing us to very easily target a specific
version only.

This is one half of #10

instead default to activate dune-common's