Make sure find_package(Sqlpp11) can be done multiple times (#306)
Make sure find_package(Sqlpp11) can be done multiple times (#306)

It is expected that CMake packages can be found multiple times without
errors. The target generated and defined by CMake, e.g. in a
*Targets.cmake file have a similar guard.
rbock/sqlpp11,rbock/sqlpp11,rbock/sqlpp11,rbock/sqlpp11