Link to core function to get more realistic test

Using umfpack_dl_solve as a test function increases the chance that the
probe will not be able to link successfully if it has not found all
libraries necessary (to link other than trivial test programs).

cmake/Modules/FindSuiteSparse.cmake

@@ -34,8 +34,9 @@ function (try_compile_umfpack varname)
   check_c_source_compiles (
         "#include <umfpack.h>
 int main (void) {
-  double t;
-  t = umfpack_timer ();
+  double info, sym, num;
+  umfpack_dl_solve (UMFPACK_A, 0, 0, 0, 0, 0, &sym, &num, &info);
+  umfpack_timer ();
   return 0;
 }" ${varname})
   cmake_pop_check_state ()