Fix MPReal detection and support.

The latest version of mpreal has a bug that breaks min/max. It also breaks with the latest dev version of mpfr. Here we add FindMPREAL.cmake which searches for the library and tests if compilation works.

Removed our internal copy of mpreal.h under unsupported/test, as it is out-of-sync with the latest, and similarly breaks with the latest mpfr. It would be best to use the installed version of mpreal anyways, since that's what we actually want to test.

Fixes #2282 (closed).