Gate the unit tests on the Boost components they link

[sebasje]

Hi,

I'm using libdjinterop for my (new) application. It's very nice and high quality, thanks a lot!

I've run into a buildsystem/dependency issue, which was easy to work around on my side, but also easy to fix in your buildsystem. I thought I'd share it. Would be nice if you could consider merging it so other users don't run into it.

The unit tests are gated on Boost_FOUND, but with CMP0167 set to NEW
(CMakeLists.txt:17), the BoostConfig package is satisfied by a headers-only
Boost install. On such a system:

Boost_FOUND=1  Boost_filesystem_FOUND=0  Boost_LIBRARIES=

The twelve test targets are configured anyway, ${Boost_LIBRARIES} expands to
nothing, and the two tests that use Boost.Filesystem fail to link with
undefined references to boost::filesystem::detail::*. The QUIET on
find_package hides the diagnostic that would have explained it, and the
else() branch ("Unit tests not available, as Boost cannot be found") is never
reached -- which is exactly the case it is there for.

This is common on Debian and Ubuntu, where libboost-dev provides the headers
and libboost-filesystem-dev is a separate package.

This PR gates the tests on Boost_filesystem_FOUND and Boost_system_FOUND as
well, and makes the message name what is missing.

Tested on Ubuntu 24.04 (CMake 3.30.5, GCC 13.3.0):

• With Boost.Filesystem installed: all 12 test targets configure and build, and
  ctest passes 12/12.
• With the Boost headers only: no test targets are configured, the build
  succeeds, and the configure output says that Boost.Filesystem/System are
  missing.

[mr-smidge]

Great spot, thank you!

I left some minor comments.

[mr-smidge]

This comment seems to focus on the code as it was, when it contained the bug that your PR fixes. But that's not the purpose of code comments - in fact, I don't think anything in this comment adds value, so would suggest removing it entirely.

Q: Was this comment written by AI?

[sebasje]

Yes, Claude code.

Note that I did review the PR and added a personal note, and of course take full responsibility for it.

Thanks for the quick review, I'll adjust and will resubmit according to your preferences.

[mr-smidge]

Claude often does this for me too!

[sebasje]

It's awfully verbose, but I have it rather tell me too much than too little.
Especially if this avoids it reverting a change some other time, because it forgot (which it also sometimes does).

Thanks btw for getting this through this quickly!

I might have some more useful (to me at least) things, nothing urgent though. Support for transactions came up, but that's obviously a larger change.

Also I'm interested in OneLibrary support. Guess we'll keep in touch! Cheers!

[sebasje]

Great spot, thank you!

I left some minor comments.

Thanks, updated the patch (squashed).