Add compile_requires

[mwoehlke]

Add an attribute to allow components to specify that a requirement should only be considered at compile-time. This is the opposite of the already existing link_requires attribute.

In particular, this allows CPS to support CMake's COMPILE_ONLY generator expression. Several likely use-cases have been cited.

[dcbaker]

This would be, for example, with something like something used with C's #embed?

[mathstuf]

It is intended to model how a project is used by, say, systemd where it will build with the headers, but dlopen the libraries before actual usage at runtime. Other places it is used:

• Python extensions should not link to libpython but instead expect the symbols to be provided by the interpreter loading the extension;
• glad loading rendering backends at runtime;
• Derelict D library projects which provide D bindings from headers, but defers loading the library to runtime;
• wi4mpi which determines the MPI to use at runtime.

[mwoehlke-kitware]

@dcbaker, maybe, but I'm not sure how; that would usually be present in a source file, not a public header.

The example I ran into is VTK, which exports a compile-only dependency on X11 (which is loaded dynamically). @mathstuf or @nickelpro might be able to give better information, but as I understand it, the likely use-case is that SomeLibrary's API exposes data types of SomeDependency, but doesn't link to SomeDependency because it is dynamically loaded at run-time instead. Something about Python was also cited; I think following that model (except in that case, it would be Python dynamically loading the library rather than the library dynamically loading Python).

[dcbaker]

Ah, that makes sense. Thanks! (and this has a +1 from me)