Configure check for sqlite3_errstr()

Previously, configure.ac checked for sqlite3_prepare_v2() and
configured Byfl to build bfbin2sqlite3 only if that function can be
found.  However, as Chris Daley (@cdaley) reports, there exist
versions of SQLite that provide sqlite3_prepare_v2() but not
sqlite3_errstr(), upon which bfbin2sqlite3 also relies.  This revision
therefore changes the configure-time check to look for
sqlite3_errstr() instead of sqlite3_prepare_v2().