Allow to search for multiple libraries in package

Some packages are split up in several library files, where each of them
doesn't have their own CMake find-module. So we cannot put this list in
the prerequisites. Unlike the headers, where we just add the directory,
all the libraries must be explicitly named on the link line. Thus, we
should allow to specify more than one name in the lib parameter.

The first name specified is designated as the "primary" library and is
assigned to the _LIBRARY variable; the others occur in _LIBRARIES as if
they were prerequisites.

Note that the build system cannot replicate this setup itself; it always
assumes that we are building (at most) one library from the source
files.
Only add option for shared libraries once

The way the test was done previously, it was added for *every* option
after `--enable-shared`, not only that one.
Added CMake find modules opm-parser and cjson
Added opm-parser-prereqs.cmake
Updated modules for Findcjson and Findopm-parser
Updated modules to find Opm-parser
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.
Changed the check_cxx_source_compiles() in Findopm-parser to look for Deck.hpp; Parser.hpp had uneccasary complicated interaction with cJSON
include the latest review comments by Roland Kaufmann