fix test for HAVE_QUAD on some instances of ubuntu 13.10

I haven't figured out why this happens, but on one of my computers
running kubuntu 13.10 g++ says the following if one tries to use
quadruple precision math:

/home/erne/src/ewoms/ewoms/common/quad.hh: In static member function ‘static quad std::numeric_limits<__float128>::min()’:
/home/erne/src/ewoms/ewoms/common/quad.hh:52:14: error: unable to find numeric literal operator ‘operator"" Q’
     { return FLT128_MIN; }

since this happens only on this machine and only with the c++ (but
not the C) compiler, i suppose that this is either an upgrade issue
with kubuntu or an issue with the ubuntu gcc stack. Strangely enough,
my other kubuntu machine does not have a problem with that...

In any case also testing for assignability of constants and using the
C++ compiler for the quadmath test won't hurt...
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
fix syntax error in configure shell script

strangly enough, it seems to work like it was in some circumstances...