Contributions to FortUTF

We need to make several contributions to FortUTF (if we want to keep using it, of course):

1. Make it possible to use it with the NAG compiler (see #10 (closed)).
2. Add cmake_minimum_required to cmake/fortutf.cmake. Version 3.17 should be enough at the moment but the author of the library might want to require 3.20+ everywhere in the project.
3. Remove ${FORTUTF_SRCS} here because it triggers the compilation of the library's source files even if the library (i.e. the target FortUTF) is already available.

There are more issues but the ones above have the highest priority in my opinion.

@b382190 @b381001