checks/check-static-exts-cross: init, move checks to separate dir

checks/sqlite-example-exts/subext.c

@@ -0,0 +1,38 @@
+#include <sqlite3ext.h>
+
+SQLITE_EXTENSION_INIT1
+
+static void sub_func(
+    sqlite3_context *ctx,
+    int argc,
+    sqlite3_value **argv
+) {
+    if (argc != 2) {
+        sqlite3_result_null(ctx);
+        return;
+    }
+
+    int a = sqlite3_value_int(argv[0]);
+    int b = sqlite3_value_int(argv[1]);
+
+    sqlite3_result_int(ctx, a - b);
+}
+
+int sqlite3_subext_init(
+    sqlite3 *db,
+    char **pzErrMsg,
+    const sqlite3_api_routines *pApi
+) {
+    SQLITE_EXTENSION_INIT2(pApi);
+
+    return sqlite3_create_function(
+        db,
+        "mysub",
+        2,
+        SQLITE_UTF8 | SQLITE_DETERMINISTIC,
+        0,
+        sub_func,
+        0,
+        0
+    );
+}