[GH #897] Use --dynext for src/glut_nci_thunks.c, fix decl, check --core

--core already has the two loader declarations in include/parrot/nci.h.
omit the duplicate dynext loader declaration, it was moved the the HEADERIZER section.
regenerate the core nci thunk c files,
do not warn on dups on glut_nci, there will be most likely duplicates, core should have none instead.