configure: Don't pre-set enable_<plugin> variables.
configure: Don't pre-set enable_<plugin> variables.

This would cause AC_SDB_PLUGIN to assume that the respective --enable-<plugin>
option was given on the command-line and, thus, behave unexpected when this is
not the case.

sysdb/sysdb,sysdb/sysdb,sysdb/sysdb,tokkee/sysdb,tokkee/sysdb,tokkee/sysdb