Reform Ewald-module cmake handling

Reform Ewald-module cmake handling

The globbing approach is convenient when all the files get the same
treatment, but sometimes inconvenient when you forget to run cmake
when adding or deleting files. Subtracting one glob from another makes
it rather hard to know what is going on, particularly with multiple
changes in flight.

Specifying the files makes it easier to see what is going on when
there are multiple different configurations that need different files,
as we are shortly likely to have as OpenCL support for PME is
integrated. This is similar to the approach now used in gpu_utils.

Change-Id: I11a62cab6fb87ed895615486a7071d420a62ea6e