Add test and check for SQLite being in single-threaded mode

tests/deny_single_threaded_sqlite_config.rs

@@ -0,0 +1,20 @@
+//! Ensure we reject connections when SQLite is in single-threaded mode, as it
+//! would violate safety if multiple Rust threads tried to use connections.
+
+extern crate rusqlite;
+extern crate libsqlite3_sys as ffi;
+
+use rusqlite::Connection;
+
+#[test]
+fn test_error_when_singlethread_mode() {
+    // put SQLite into single-threaded mode
+    unsafe {
+        // 1 == SQLITE_CONFIG_SINGLETHREAD
+        assert_eq!(ffi::sqlite3_config(1), ffi::SQLITE_OK);
+        println!("{}", ffi::sqlite3_mutex_alloc(0) as u64);
+    }
+
+    let result = Connection::open_in_memory();
+    assert!(result.is_err());
+}