create iso_week header only library in cmake-file

[allspark]

No description provided.

[lrusak]

any updates here? it would be nice to have the iso_week.h header installed via cmake

[mellery451]

I think the cmake files have diverged a bit and this PR would need to be rebased/updated if it were to get merged. That said, there are a few questionable changes (Threads ONLY for MSVC seems odd, for example...) that I think would need review before including. Also, I wonder if instead of JUST iso_week, there should be a "date extras" interface library that includes all the other interesting headers that aren't part of core date. We'd probably need @HowardHinnant to advise, but from a simplicity point of view, it seems like bundling all the optional/additional headers into an "extras" target would be simplest. The only downside is if one or more of these headers brings in additional dependencies - that just complicates things. We might also need this extra target to be optional in case some packagers don't want to include it.

[HowardHinnant]

A "date extras" interface that includes everything sounds good to me. All of the extra headers are header-only libraries, even if they depend on tz.h. And they don't introduce any other dependencies.

Looking at this outside of CMake, all it takes to use any of these extras is to simply include it. No source file needs to be compiled. No extra utilities need to be downloaded (not even the IANA tz database).

[mellery451]

sounds good - I'll see if I can formulate a patch that creates the "extras" target for cmake users.