In case the support is not available, just stop building the
libbsd-ctor.a library, which is a nice to have thing, but should not
have been a hard requirement from the start. This should allow to
build libbsd on non-glibc based systems using another libc.
This is a wrapper over the glibc fopencookie() function.
We diverge from the FreeBSD, OpenBSD and DragonFlyBSD declarations,
because seekfn() there wrongly uses fpos_t, assuming it's an integral
type, and any code using that on a system where fpos_t is a struct
(such as GNU-based systems or NetBSD) will fail to build. In which case,
as the code has to be modified anyway, we might just as well use the
correct declaration.
