Autotools relayout

Dear Norbert,

This branch moves around the sources (obj/MPFR -> src/MPFR) and built object files (obj/MPFR -> obj) to enable 1. out-of-tree builds and 2. to re-enable extensible backend support. Incidently, also the `make check` target is not part of `all` anymore as that's a non-standard configuration. Besides there have been no behavioural changes.

If you agree, please pull the changes into `master`. If there is anything left to fix, please let me know.

Franz