CMake tweaks for non-sqlcipher key support

The `SQLITE_HAS_CODEC` option currently does two separate things: it
enables usage of the key API, and it forces searching for sqlcipher
headers when finding SQLite.

When attempting to use an alternative to sqlcipher (such as sqlite-ee or
sqlite-multiple-ciphers) the former is wanted, and the latter is
absolutely not wanted.

The previous commit allows working around the sqlcipher search by simply
bypassing the find process entirely and allowing a parent project to set
up SQLite3::SQLite however it wants.  That, combined with
SQLITE_HAS_CODEC=ON, allows using SQLiteCpp with encryption API with
some alternative to sqlcipher.

This commit adds some small "niceness" updates:

- make the SQLITE_HAS_CODEC compile definition PRIVATE as it only
  affects internal code in Database.cpp and does not require propagation
  to linking code.  (Building sqlcipher itself requires this definition,
  but SQLiteCpp does not support building sqlcipher, it only finds an
  already-built one).
- sqlite3_key_v2 is not actually required or used (and appears to be
  sqlcipher-specific), but sqlite3_rekey is.  (`sqlite3_key` and
  `sqlite3_rekey` are supported by all of sqlcipher, sqlite-mc, and
  sqlite-ee).
- SQLITE_HAS_CODEC combined with SQLITECPP_INTERNAL_SQLITE will not
  compile as the internal version is stock sqlite3, and this adds a
  check and fatal error if attempting that configuration.

Author: jagerman

CMakeLists.txt

@@ -228,11 +228,12 @@ if (SQLITE_ENABLE_ASSERT_HANDLER)
     target_compile_definitions(SQLiteCpp PUBLIC SQLITECPP_ENABLE_ASSERT_HANDLER)
 endif (SQLITE_ENABLE_ASSERT_HANDLER)
 
-option(SQLITE_HAS_CODEC "Enable database encryption API. Not available in the public release of SQLite." OFF)
+option(SQLITE_HAS_CODEC "Enables usage of the Database key() and rekey() methods. Requires a custom SQLite providing the key API such as sqlcipher/sql-ee/sqlite-multi-ciphers" OFF)
 if (SQLITE_HAS_CODEC)
-    # Enable database encryption API. Requires implementations of sqlite3_key & sqlite3_key_v2.
-    # Eg. SQLCipher (libsqlcipher-dev) is an SQLite extension that provides 256 bit AES encryption of database files. 
-    target_compile_definitions(SQLiteCpp PUBLIC SQLITE_HAS_CODEC)
+    # Enable database encryption API. Requires implementations of sqlite3_key & sqlite3_rekey.
+    # SQLCipher, SQLite Multiple Ciphers, and the commercial SQLite Encryption Extensions provide
+    # these API functions for encrypting a database.
+    target_compile_definitions(SQLiteCpp PRIVATE SQLITE_HAS_CODEC)
 endif (SQLITE_HAS_CODEC)
 
 option(SQLITE_USE_LEGACY_STRUCT "Fallback to forward declaration of legacy struct sqlite3_value (pre SQLite 3.19)" OFF)
@@ -279,8 +280,12 @@ endif ()
 ## Build provided copy of SQLite3 C library ##
 
 option(SQLITECPP_INTERNAL_SQLITE "Add the internal SQLite3 source to the project." ON)
-option(SQLITECPP_FIND_SQLITE "Attempt to find SQLite via find_package (requires SQLITECPP_INTERNAL_SQLITE=OFF).  If disabled then a SQLite3::SQLite cmake target must already exist." ON)
+option(SQLITECPP_FIND_SQLITE "Attempt to find SQLite (or sqlcipher, with SQLITE_HAS_CODEC) automatically. Requires SQLITECPP_INTERNAL_SQLITE=OFF. If disabled then a SQLite3::SQLite cmake target must already exist." ON)
 if (SQLITECPP_INTERNAL_SQLITE)
+    if (SQLITE_HAS_CODEC)
+        message(FATAL_ERROR "Internal sqlite3 source does not support SQLITE_HAS_CODEC")
+    endif()
+
     message(STATUS "Compile sqlite3 from source in subdirectory")
     option(SQLITE_ENABLE_RTREE "Enable RTree extension when building internal sqlite3 library." OFF)
     option(SQLITE_ENABLE_DBSTAT_VTAB "Enable DBSTAT read-only eponymous virtual table extension when building internal sqlite3 library." OFF)

meson_options.txt

@@ -4,8 +4,9 @@
 option('SQLITE_ENABLE_COLUMN_METADATA', type: 'boolean', value: false, description: 'Enable Column::getColumnOriginName(). Require support from sqlite3 library.')
 ## Enable the user definition of a assertion_failed() handler (default to false, easier to handler for beginners).
 option('SQLITE_ENABLE_ASSERT_HANDLER', type: 'boolean', value: false, description: 'Enable the user definition of a assertion_failed() handler.')
-## Enable database encryption API. Requires implementations of sqlite3_key & sqlite3_key_v2.
-## Eg. SQLCipher (libsqlcipher-dev) is an SQLite extension that provides 256 bit AES encryption of database files. 
+## Enable database encryption API. Requires implementations of sqlite3_key & sqlite3_rekey.
+## SQLCipher, SQLite Multiple Ciphers, and the commercial SQLite Encryption Extensions provide
+## these API functions for encrypting a database.
 option('SQLITE_HAS_CODEC', type: 'boolean', value: false, description: 'Enable database encryption API. Not available in the public release of SQLite.')
 ## Force forward declaration of legacy struct sqlite3_value (pre SQLite 3.19)
 option('SQLITE_USE_LEGACY_STRUCT', type: 'boolean', value: false, description: 'Fallback to forward declaration of legacy struct sqlite3_value (pre SQLite 3.19)')