Add option USE_RUNPATH to CMake build system

This PR adds the option USE_RUNPATH to the build system. If USE_RUNPATH is set to ON the path to various shared libraries will be embedded in the binaries; this way a binary can be invoked without fiddling with LD_LIBRARY_PATH and such. 

The python distribution still needs an external method (i.e. convention which is currently applied, LD_LIBRARY_PATH or ld.so.conf) to locate the shared libraries.

The default setting of USE_RUNPATH is OFF.