Internal test checking for c++1X but not c++2X

Referencing this: https://github.com/LLNL/blt/blob/develop/tests/internal/CMakeLists.txt#L246
```
if (ENABLE_CUDA AND ENABLE_MPI AND
"${BLT_CXX_STD}" MATCHES c\\+\\+1)
...
```

Open tests/internal/CMakeLists.txt around line 246 and inspect the C++ standard matching condition shown in the issue. Confirm how the condition handles c++1X and c++2X, then update the check so the intended standard is recognized and run the relevant internal CMake test to verify it.

Written by the indexing model from the issue text.