CTEST_CUSTOM_* parameter have to be put in a CTestCustum.cmake file which itself has to be in the build directory

So this is the come-back of btest target, and the default target is empty again.

* the dashboard is there: http://my.cdash.org/index.php?project=Eigen
* now you can run the tests from the top build dir
  and submit report like that (from the top build dir):
  ctest -D Experimental
* todo:
 - add some nighlty builds (I'll add a few on my computer)
 - add valgrind memory checks, performances tests, compilation time tests, etc.