build system: prevent endless loop when using dunecontrol

this seems to be a bug in cmake 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
configure script: also delete the cmake cache

This replicates the autotools behavior and was suggested by Roland
Kaufmann.
configure script: add a stub implementation for the --config-cache argument

so far, it is only a flag which prevents the cache from being
deleted. To warn the user about a potential infinite loop, a warning
is printed if this flag was specified and some compiler or some compiler
flags were set by the user. This idea was proposed by Roland Kaufmann.
include the latest review comments by Roland Kaufmann
Merge pull request #430 from andlaus/fix_infinite_configure

build system: prevent endless loop when using dunecontrol