(cmake branch) code is not compatible with CMake 3.18

[wavepacket]

Again a rather small issue with two possible fixes. I happened to fall into it while trying to compile the tensor library on an older Debian with exactly CMake 3.18.x

In TensorOptions.cmake, there is a generator exprssion like $CONFIG:Release,RelWithDebInfo.
According to the CMake documentation, this expression is only allowed from CMake 3.19 onwards. CMake 3.18 only allows a single configuration to be specified.

There are principally two solutions. None is perfect IMO.

• You could rewrite this generator expression with an $OR:... and two CONFIG expressions. This is ugly.
• You could stop pretending about the compatibility and require CMake 3.19.