Introduce a 'native' feature

The 'native' feature forces usage of the system sqlite, failing to
compile if it is absent. This is distinct from the defaulting that the
bundled sources as not used as fallback.

Depends-On: stainless-steel/sqlite3-sys#10

Author: WhyNotHugo

Cargo.toml

@@ -38,6 +38,7 @@ bundled = ["sqlite3-sys/bundled"]
 extension = []
 encryption = ["sqlite3-sys/encryption"]
 linkage = ["sqlite3-sys/linkage"]
+native = ["sqlite3-sys/native"]
 
 [dependencies.sqlite3-sys]
 version = "0.18"