PROTON-2773: add PN_FALLTHROUGH macro ready for c++17/c21

[jiridanek]

No description provided.

[jiridanek]

The original motivation for this is to be able to have composite CMake build with skupper-router. With skupper-router adding proton as cmake subproject. To do this, both projects need to compile with the same flags, and skupper-router defines -Wimplicit-fallthrough. Therefore, proton should be clean for this warning too.

[jiridanek]

See also https://lore.kernel.org/lkml/a588204afbfe4c8dd56d0cb00d8e6e14dc561a62.camel@perches.com/

where Joe Perches who maintains the Linux kernel's patch validator (scripts/checkpatch.pl) wrote a tool to convert all of the Linux kernel's comments into attribute((fallthrough)), but also provided statistics of every possible spelling of fallthrough seen in the kernel, which is useful data for those looking to fix every possible case.

• False positive -Wimplicit-fallthrough warning in flex-generated C code llvm/llvm-project#42810 (comment)

[astitcher]

Basically lgtm.
My comments are basically stylistic, just my preference for if...else if.. chains in this case.

[astitcher]

again could be #elif

[jiridanek]

no, because we might be on older c that has a __has_c_attribute but not the [[fallthrough]];, so we would go into

#if !defined PN_FALLTHROUGH && defined __STDC_VERSION__ && defined __has_c_attribute

but not define PN_FALLTHROUGH there

[astitcher]

this could be #else

[jiridanek]

Thanks for looking into this. I changed it to #if#elif where it was straightforward to do.