maybe we removed the last n characters

    (LT(PL * 5,DO) ERROR('079','USER IS NOT SUFFICIENTLY POLITE','1'))
    (GT(PL * 3,DO) ERROR('099','USER IS OVERLY POLITE','1'))

Finally, we know how polite to be.

If you want to have a library that can also be a standalone executable, just put the main function in an extra file and don’t compile that file when using the library as a library.
You could also use the preprocessor to do it similar to python but please don’t.

Just use any build tool, and have two targets, one library and one executable:

LIB_SOURCES = tools.c, stuff.c, more.c
EXE_SOURCES = main.c, $LIB_SOURCES

Edit: added example

It might also introduce spurious data dependencies

Those need to be in the in smallest cache or a register anyway. If they are in registers, a modern, instruction reordering CPU will deal with that fine.

to store a bit you now need to also read the old value of the byte that it’s in.

Many architectures read the cache line on write-miss.

The only cases I can see, where byte sized bools seems better, are either using so few that all fit in one chache line anyways (in which case the performance will be great either way) or if you are repeatedly accessing a bitvector from multiple threads, in which case you should make sure that’s actually what you want to be doing.