Make it possible to disable bundled SQLite

Also disable it in CMakeLists.txt which is used to install libdeltachat
system-wide.

Cargo.toml

@@ -56,7 +56,7 @@ r2d2 = "0.8.9"
 r2d2_sqlite = "0.18.0"
 rand = "0.7.0"
 regex = "1.1.6"
-rusqlite = { version = "0.25", features = ["bundled"] }
+rusqlite = "0.25"
 rust-hsluv = "0.1.4"
 rustyline = { version = "4.1.0", optional = true }
 sanitize-filename = "0.3.0"
@@ -115,8 +115,9 @@ name = "search_msgs"
 harness = false
 
 [features]
-default = []
+default = ["bundled-sqlite"]
 internals = []
 repl = ["internals", "rustyline", "log", "pretty_env_logger", "ansi_term", "dirs"]
 vendored = ["async-native-tls/vendored", "async-smtp/native-tls-vendored"]
 nightly = ["pgp/nightly"]
+bundled-sqlite = ["rusqlite/bundled"]