lib: Ensure we use _GNU_SOURCE in enum templates

lib: Ensure we use _GNU_SOURCE in enum templates

Due to the way glib-mkenums runs the preprocessor itself, it
doesn't pick up the `AC_USE_SYSTEM_EXTENSIONS()` that we have in
`configure.ac`.

This blew up in an obscure way when I later wanted to `#include
"libglnx.h"` in one of the headers, since it needs the `basename()`
from `string.h` which is only available with `_GNU_SOURCE`.

Closes: #616
Approved by: jlebon