configure: fix AC_CACHE_VAL warnings on Fedora 10

configure: fix AC_CACHE_VAL warnings on Fedora 10

Autoconf 2.63 (and maybe earlier releases) complains about the cache
variable name used in aclocal/libblkid.m4:

configure.ac:217: warning: AC_CACHE_VAL(libblkid_is_recent, ...):
suspicious cache-id, must contain _cv_ to be cached
../../lib/autoconf/general.m4:1974: AC_CACHE_VAL is expanded from...
aclocal/libblkid.m4:2: AC_BLKID_VERS is expanded from...
configure.ac:217: the top level

This addresses
  https://bugzilla.redhat.com/attachment.cgi?bugid=481386 .

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>