review updates:

- unused variable
- correct return code
- consistent ifdef check
- don't use c++17

Co-authored-by: Yu-Hsiang M. Tsai <yhmtsai@gmail.com>
Co-authored-by: Pratik Nayak <pratik.nayak@kit.edu>
Co-authored-by: Tobias Ribizel <ribizel@kit.edu>
Co-authored-by: Thomas Grützmacher <thogru.kit@gmx.de>

CMakeLists.txt

@@ -248,7 +248,6 @@ if(GINKGO_BUILD_MPI)
         ${Ginkgo_SOURCE_DIR}/cmake/openmpi_test.cpp
                 COMPILE_DEFINITIONS -DCHECK_HAS_OPEN_MPI=1
         LINK_LIBRARIES MPI::MPI_CXX
-                OUTPUT_VARIABLE output
         )
     if(uses_openmpi)
         try_compile(valid_openmpi_version

cmake/openmpi_test.cpp

@@ -39,12 +39,12 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 int main()
 {
 #if CHECK_HAS_OPEN_MPI && defined(OPEN_MPI) && OPEN_MPI
-    static_assert(true);
-#elif CHECK_OPEN_MPI_VERSION
+    static_assert(true, "Check availability of OpenMPI");
+#elif CHECK_OPEN_MPI_VERSION && defined(OPEN_MPI) && OPEN_MPI
     static_assert(OMPI_MAJOR_VERSION > 4 ||
-                  (OMPI_MAJOR_VERSION == 4 && OMPI_MINOR_VERSION >= 1));
+                      (OMPI_MAJOR_VERSION == 4 && OMPI_MINOR_VERSION >= 1),
+                  "Check OpenMPI version.");
 #else
-    static_assert(false);
+    static_assert(false, "No OpenMPI available");
 #endif
-    return 1;
 }