[cmake] Introduce `-Ddev=On` with preferred developer settings:

Reduce build times:
  - use lld or gold or good-old-ld, in that order of preference;
  - split debug info to accelerate rebuilding and relinking;
  - do not rebuild shared libraries just because a shared lib they depend on has changed;
  - generate the gdb-index to accelerate root startup in gdb.
Turn on -Werror to not overlook warnings.