* Avoid writing the full path in enumtypes.h
Helps reproducible builds, as build paths can vary between builds.
* Use g_intern_static_string() for enum/flags type names
We are emitting a static string, so use that shortcut function to
intern it, which will save one allocation.
* Use g_once_init_enter/leave to register enum/flags type names
This is the more modern way, and ensures thread-safety.