pkg-config generation fails with `WITH_BOOST=OFF`

[planetmarshall]

I didn't address this in my PR (#501) - but the pkg config generation script doesn't work as it requires all of the target properties to be resolved at configuration time, whereas many of the properties (specifically for the std::filesystem configuration) are only resolved at build time, so the resulting h5cpp.pc file just contains generator expressions.

If you want to generate a configuration file with generator expressions you need to use file(GENERATE...)

[eugenwintersberger]

This entire things requires some review.

[planetmarshall]

It's up to you of course but if this were my project, I might consider abandoning direct pkg-config support entirely and just supporting CMake. I guess it depends what your downstream users require.

[jkotan]

Pkg-config is required by our users in DESY. Therefore, we should keep it

[eugenwintersberger]

pkg-config is extremely common in the Linux world and can be used by virtually any build system. I agree that CMake package files are far more powerful but the only build system supporting them, aside from cmake, is meson. So I think supporting pkg-config is currently absolutely necessary. Let's see how things evolve in future and maybe one day we can get rid of it ;)

What we definitely have to improve is the way the files get built. I hope I can merge Andrews PR branch into my current working branch and then do a merge with master next weekend. Afterwards a review of the entire build system is I guess a good thing to do. But first I would like to read through Andrews changes.

[jkotan]

@planetmarshall thank you for the report. I've decided to change generator expressions to if statements because linker options are visible during cmake configuration and the solution is much easier