g++ 13.x is confused by the reinterpret_cast in Kakoune's memory.hh
allocator. Use -Wno-stringop-overflow to silence a large number of
verbose false alarms.

We also discussed using "git clean -dXf" but that could remove files
that were not generated by make.

Closes #4619

Merge all lookarounds into the same instruction, merge splits, merge
literal ignore case with literal...

Besides reducing the amount of almost duplicated code, this improves
performance by reducing pressure on the (often failing) branch target
prediction for instruction dispatching by moving branches into the
instruction code themselves where they are more likely to be well
predicted.

Fixes #3985

Add that libexec directory to the PATH instead of the current kak
binary directory to avoid impacting other commands.

The libexec directory currently only contains a symlink back to
the Kakoune binary.