BITCOIN_SUBDIR_TO_INCLUDE no longer works #20118

I haven't been able to figure out why yet, but it seems gcc/clang/g++/clang++ -E no longer does the same thing as cpp

0.20 seems unaffected, so something else is apparently going on.

Investigating further, it looks like 0.20 is indeed also broken, and only Knots 0.20 is fine... and it's because I already fixed this years ago in #5872 which nobody cared to review. Sigh.

(Apparently g++ -E only works with filename inputs, whereas cpp accepts stdin, but BITCOIN_SUBDIR_TO_INCLUDE uses a conftest.cpp file, so that turned out to be a non-issue)

Maybe consider https://github.com/theuni/bitcoin/commit/2ef8a6fc3b1de989702adb04ebb066bd101350f0 ?

I'm going to close this as wont-fix.

BITCOIN_SUBDIR_TO_INCLUDE is only used for BDB detection, and only in certain cases. Given that using BDB is (being) deprecated, there is somewhat less reason to fix this. This is compunded by the fact that if this has indeed been broken for 2 years, and not a single other person has realised / opened an issue, clearly BITCOIN_SUBDIR_TO_INCLUDE is not getting much actual use, and isn't regularly breaking anyones ability to build (with BDB).

Maybe consider https://github.com/theuni/bitcoin/commit/2ef8a6fc3b1de989702adb04ebb066bd101350f0 ?

Could be a (much simpler to review and grok) option, but so could just removing this code entirely, if it is completely broken, given the point above.