Update to mpfr-2.3.1.  Add new distribution patch, appease pkglint.

Changes from version 2.3.0 to version 2.3.1:

- Changes in the behavior of mpfr_strtofr and in its documentation
  concerning particular cases where the code and the documentation
  did not match.
- Bug fixes; see <http://www.mpfr.org/mpfr-2.3.0/#bugs>.
- Configure test for TLS support.
- Improved MPFR manual.

(bjs)