[bugfix,cmake] allow cmake to parse dune.module files containing shell snippets

The duncontrol / dune.module mechanism allowes to overload the run_* commands, which
are executed in order to setup/build/etc. a module. This patch fixes a bug in the cmake
parsing of the dune.module file. Somehow cmake tried to interpret the content of the
dune.module as cmake code. We now use a different strategy to parse the file and avoid
storing the content in a temporary variable.

>> please backport to 2.4