Add CMake standard

[msleigh]

First version of a CMake standard.

This is a rough/incomplete first version to get things started. Any serious problems should be addressed in this PR but desirable but non-critical refinements, additions, improvements, etc. can be dealt with in the usual way (create a GitHub Issue).

[cresswellp]

There are references to a few ifs-source functions - ifs_target_fortran_module_directory, ifs_target_compile_definitions_FILENAME, add_symlink - that are described as if they apply everywhere.

Although I've often wondered if add_symlink should be added into ecbuild; I don't think any other projects have anything similar but it has clear advantages over the CMake equivalent.