Master branch update to remove CMake dep `Boost::static_assert`

[chandryan]

On MSM's master branch (which depends on Boost.serialization), the CI is currently failing due to a missing Boost::static_assert target: https://github.com/boostorg/msm/actions/runs/28042133791/job/83010826498#step:5:43

I assume this library needs its master branch updated, is that correct?
Since this repo's master branch hasn't been touched for a while, I thought it might be useful to point out the potential need to update it for the upcoming Boost release.

Although a master branch update might just fix a symptom – I don't understand why it fails on MSM's CI but not in the superproject's, seeing there still is a CMake stub target for static_assert I guess it should not have broken.

FYI @Lastique @pdimov

[Lastique]

I assume this library needs its master branch updated, is that correct?

Yes, in particular #340 needs to be merged to master. @pdimov @robertramey could you merge to master, please?

Although a master branch update might just fix a symptom – I don't understand why it fails on MSM's CI but not in the superproject's, seeing there still is a CMake stub target for static_assert I guess it should not have broken.

I'm not sure what exactly is being done in the superproject CI, but there are two points to consider:

1. The superproject CI doesn't build or run library tests. Which means if the dependency is only through tests, it will not be triggered.
2. The superproject checks out all libraries, including Boost.StaticAssert, which defines the static_assert target. So even if the library itself depends on it (like Boost.Serialization's master), that target will be found. This is unlike MSM's CI, which only checks out libraries that are needed according to dependencies from source code.