Fix caching of --version-script check

Fix caching of --version-script check

The shell code in AC_CACHE_CHECK() should set the variable that we say
is being cached.  So set ac_cv_version_script when testing whether ld
accepts --version-script, and then set LIBIBVERBS_VERSION_SCRIPT based
on the (possibly cached) value of ac_cv_version_script outside of the
AC_CACHE_CHECK().

Signed-off-by: Roland Dreier <rolandd@cisco.com>