bug#25680: maint: tweaks so syntax tests pass for previous commit

[Paul Eggert]

I see some problems with this followup patch:

http://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=bd4bb42d65aac6591277066739ca42d1ddcc2d0e

in that it will make force-link.c and force-link.h harder to move to 
gnulib. Is there some way that we can have the code pass syntax checks 
without tying it so tightly to coreutils?

Also, why do the syntax checks require those redundant 'extern's? 
They're not that common in practice and I'd rather avoid them.