Add SQLITECPP_FIND_SQLITE to disable find_package

Currently the SQLite3 code is quite limited, and really only has two
options:
- build bundled sqlite3 version
- use find_package() to find it

This commit adds an option to enable a third case: letting an external
cmake project worry about finding or building and providing sqlite by
setting `SQLITECPP_FIND_SQLITE` to `OFF`.  This requires that the parent
project has already set up some SQLite3::SQLite cmake target that
SQLiteCpp can then simply use without trying to find anything.

This is particularly useful when building a custom sqlite3 (such as
SQLite3 Multiple Ciphers) which will not be found via `find_package`.
Currently you can *somewhat* hack around this by setting a bunch of
cache variables to make the find_package short-circuit, but that feels
inelegant.

Author: jagerman

CMakeLists.txt

@@ -279,14 +279,15 @@ 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 SQLite::SQLite3 cmake target must already exist." ON)
 if (SQLITECPP_INTERNAL_SQLITE)
     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)
     # build the SQLite3 C library (for ease of use/compatibility) versus Linux sqlite3-dev package
     add_subdirectory(sqlite3)
     target_link_libraries(SQLiteCpp PUBLIC SQLite::SQLite3)
-else (SQLITECPP_INTERNAL_SQLITE)
+elseif (SQLITECPP_FIND_SQLITE)
     # When using the SQLite codec, we need to link against the sqlcipher lib & include <sqlcipher/sqlite3.h>
     # So this gets the lib & header, and links/includes everything
     if(SQLITE_HAS_CODEC)
@@ -332,7 +333,14 @@ else (SQLITECPP_INTERNAL_SQLITE)
             set_target_properties(SQLiteCpp PROPERTIES COMPILE_FLAGS "-DSQLITECPP_HAS_MEM_STRUCT")
         endif()
     endif()
-endif (SQLITECPP_INTERNAL_SQLITE)
+else ()
+    if (NOT TARGET SQLite::SQLite3)
+        message(FATAL_ERROR "SQLITECPP_FIND_SQLITE=OFF requires a pre-existing SQLite::SQLite3 cmake target")
+    endif()
+
+    message(STATUS "Using pre-existing SQLite::SQLite3 cmake target")
+    target_link_libraries(SQLiteCpp PUBLIC SQLite::SQLite3)
+endif ()
 
 ## disable the optional support for std::filesystem (C++17)
 option(SQLITECPP_DISABLE_STD_FILESYSTEM "Disable the use of std::filesystem in SQLiteCpp." OFF)