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.

unsupported/test/CMakeLists.txt

@@ -55,13 +55,11 @@ ei_add_test(FFT)
 
 ei_add_test(EulerAngles)
 
-find_package(MPFR 2.3.0)
-find_package(GMP)
-if(MPFR_FOUND AND EIGEN_COMPILER_SUPPORT_CPP11)
-  include_directories(${MPFR_INCLUDES} ./mpreal)
+find_package(MPREAL)
+if(MPREAL_FOUND AND EIGEN_COMPILER_SUPPORT_CPP11)
   ei_add_property(EIGEN_TESTED_BACKENDS "MPFR C++, ")
-  set(EIGEN_MPFR_TEST_LIBRARIES ${MPFR_LIBRARIES} ${GMP_LIBRARIES})
- ei_add_test(mpreal_support "-std=c++11" "${EIGEN_MPFR_TEST_LIBRARIES}" )
+  include_directories(${MPREAL_INCLUDES})
+  ei_add_test(mpreal_support "-std=c++11" "${MPREAL_LIBRARIES}" )
 else()
   ei_add_property(EIGEN_MISSING_BACKENDS "MPFR C++, ")
 endif()

unsupported/test/mpreal_support.cpp

@@ -1,3 +1,4 @@
+#include <mpreal.h>  // Must be included before main.h.
 #include "main.h"
 #include <Eigen/MPRealSupport>
 #include <Eigen/LU>