Log Message:
Move pdo_sqlite3 dependency to under sqlite option.

Previously, I added a dependency on pdo_sqlite3 because an owncloud
instance failed to run without PDO.  This commit moves the dependency
to be conditional on the sqlite option, so that it doesn't apply if
mysql is selected.

The upstream method selection code is hard to follow, and in
particular I don't understand how dbtype=sqlite is expected to use
MDB2 and dbtype=sqlite3 PDO.  So with the sqlite option both chunks of
code are availale.  I suspect there is an upstream bug lurking, but I
can't point to it.