Log Message:
Updated mpfr to 3.1.6.

Changes from version 3.1.5 to version 3.1.6:
- Improved MPFR manual.
- Bug fixes (see <http://www.mpfr.org/mpfr-3.1.5/#fixed> and ChangeLog file).
- Autotools: Under Linux, make sure that the old dtags (when supported)
  are used if LD_LIBRARY_PATH is defined; otherwise "make check" would
  check an installed, compatible MPFR library found in LD_LIBRARY_PATH
  instead of the one that has been built with "make".