configure: add --enable-rpath

This option facilitates testing shared libarary builds: for instance
fate builders do no longer need to set LD_LIBRARY_PATH as the binaries will
get the right search paths hardcoded into their executable file.

This option is only meant to be used for testing purposes: The installed
libraries must not move around in the file system, and doing so will
cause a lot of subtle problems. For more information why using RPATH is
dangerous, please refer to

https://blog.flameeyes.eu/2010/06/the-why-and-how-of-rpath