Add ability to use SQLCipher when adding GRDB through SPM

[giedreblekaite]

Hello,

We are adding GRDB as a package through Swift Package Manager and we noticed that we cannot use SQLCipher instead of GRDBSQLite due to SWIFT_PACKAGE being defined. Would it be possible to change these conditional compilations to allow using SQLCipher in GRDB package?

[morajabi]

@giedreblekaite See #1495