HDF5: Warn if SWMR is requested but unavailable

dune/fufem/hdf5/hdf5file.hh

@@ -42,10 +42,15 @@ public:
     bool const fileExists = stat(filename.c_str(), &buffer) == 0;
 
     unsigned int flags = fileExists ? H5F_ACC_RDWR : H5F_ACC_EXCL;
+    if (swmr) {
 #if H5_VERSION_GE(1, 10, 0)
-    if (swmr)
       flags |= H5F_ACC_SWMR_WRITE;
+#else
+      std::cerr
+          << "WARNING: SWMR requested but not supported by this version of HDF5"
+          << std::endl;
 #endif
+    }
 
     file_ = fileExists ? H5Fopen(filename.c_str(), flags, fapl)
                        : H5Fcreate(filename.c_str(), flags, H5P_DEFAULT, fapl);
@@ -69,6 +74,10 @@ public:
 #if H5_VERSION_GE(1, 10, 0)
     swmrEnabled_ = true;
     H5Fstart_swmr_write(file_);
+#else
+    std::cerr
+        << "WARNING: SWMR requested but not supported by this version of HDF5"
+        << std::endl;
 #endif
   }