autogenerating DEFAULT_CONFIG via CMAKE

The DEFAULT_CONFIG preprocessor #define is now generated
based on the CMAKE_INSTALL_PREFIX of cmake. This will make
sure, that the config file is located under the same
hirarchy as the executable.
added config installation

The umurmur.example.config file gets installed into
${CMAKE_INSTALL_PREFIX}/etc so that it is automagically
found when starting umurmurd. Note, that the config file
is only installed when there is no old config file found
in this spot.
fixed a typo