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.
