The [cmake documentation of `CXX_STANDARD_REQUIRED`](https://cmake.org/cmake/help/v3.5/prop_tgt/CXX_STANDARD_REQUIRED.html)
demonstrate that this was previously incorrectly set. This forces the
compiler flag to be specified. In the case of gcc 4.8 it sets
`-std=c++1y`. Newer gcc (since version 5) will sets `-std=c++14`.

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Re #21330
When uing gcc/g++ toolchain, cmake will now fail if the versions of gcc
and g++ do not match.

use the most conservative as a safe choice

Merge to ensure CrystalFieldFunction is available and up to date.

The options can now be edited on a per-target basis later in the
cmake process.
Refs #0

Also adds a __cplusplus check in the code.