Add infrastructure for C++26 compiler support (GCC16)

[jbcoe]

No description provided.

[Twon]

Looks good, a few minor comments but nothing major

[jbcoe]

Looks good, a few minor comments but nothing major

I guess in general we want noisy failure when reflection is not supported rather than silent fallback. Can I change the CMake setup to ensure this?

[Twon]

Looks good, a few minor comments but nothing major

I guess in general we want noisy failure when reflection is not supported rather than silent fallback. Can I change the CMake setup to ensure this?

I think the fatal error is sufficient once we avoid specific compiler flags or restrict them to the appropriate compiler, as then we avoid failures due to using an unsupported flag and as then it should just work as future compilers add support.

[nbx8]

@jbcoe, one suggestion (not essential but could be helpful): split the C++26 compiler support and the sandbox option update into two separate PRs.