Get rid of EIGEN_TEST_FUNC, unit tests must now be declared with EIGEN_DECLARE_TEST(mytest) { /* code */ }.

This provide several advantages:
- more flexibility in designing unit tests
- unit tests can be glued to speed up compilation
- unit tests are compiled with same predefined macros, which is a requirement for zapcc

unsupported/test/mpreal_support.cpp

@@ -7,7 +7,7 @@
 using namespace mpfr;
 using namespace Eigen;
 
-void test_mpreal_support()
+EIGEN_DECLARE_TEST(mpreal_support)
 {
   // set precision to 256 bits (double has only 53 bits)
   mpreal::set_default_prec(256);