build system: prevent endless loop when using dunecontrol

Ha! found it:

this seems to be a bug in my cmake (i.e., 2.8.10.2): if the user sets the
CMAKE_CXX_COMPILER variable for a build directory where this variable
has already been set, one gets an endless loop. This stings especially
if using the dunecontrol compatibility layer as the compiler flags are
unconditionally set via the CXX_FLAGS environment variable in the
options file. Running duncontrol on a module twice will thus trigger the
infinite loop if some compiler flags are set by the user.

The solution is relatively simple: Delete the CMakeFiles directory
before calling cmake. for the dunecontrol compatibility mode, we do
this in the configure script. For details about the cmake bug, see
http://www.cmake.org/Bug/view.php?id=14119