These are now maintained in the separate "lv2-examples" project to keep the
dependencies of lv2 itself minimal.

This shouldn't be an issue in practice, but avoids a potential mistake if the
forge was to be used again without being properly reset, which clang-tidy 20
gained the ability to recognize.

Things get confused when these flags differ across projects, so universally use
"system" for external dependencies.

It shouldn't normally be possible for this to happen, but this change makes it
statically impossible.