Don't use system SQLite if headers not found.

Gentoo multilib environments may have situations when there is no 64-bit SQLite package installed but filesystem containts 32-bit libsqlite3.so and we have `LIBSQLITE3_LIBRARY` set, but not `LIBSQLITE3_INCLUDE_DIR` and `LIBSQLITE3_FOUND`. Don't try to use system SQLite headers if they are not present.

Fixes #3108.