gnulib: add getopt module

gnulib: add getopt module

Unconditionally use gnulib's getopt module. This is needed by the bhyve driver
to provide a reentrant interface for getopt.

Several gnulib headers rely on features.h being included by ctype.h to provide
__GNUC_PREREQ, but on systems without glibc, this is not provided. In these
cases __GNUC_PREREQ gets redefined to 0, which causes build errors from checks
in src/internal.h.
Therefore, define __GNUC_PREREQ as early as possible. config-post.h is probably
the first header that is included, before any other headers.