Check for rb_enc_interned_str_cstr and fall back

Truffle Ruby doesn't implement this function yet, so we need to check
for it and then fall back if it's not there.

ext/sqlite3/extconf.rb

@@ -109,6 +109,10 @@ def configure_extension
 
         abort_could_not_find(libname) unless find_library(libname, "sqlite3_libversion_number", "sqlite3.h")
 
+        # Truffle Ruby doesn't support this yet:
+        # https://github.com/oracle/truffleruby/issues/3408
+        have_func("rb_enc_interned_str_cstr")
+
         # Functions defined in 1.9 but not 1.8
         have_func("rb_proc_arity")
 

ext/sqlite3/statement.c

@@ -364,6 +364,23 @@ column_count(VALUE self)
     return INT2NUM(sqlite3_column_count(ctx->st));
 }
 
+#if HAVE_RB_ENC_INTERNED_STR_CSTR
+static VALUE
+interned_utf8_cstr(const char * str)
+{
+    return rb_enc_interned_str_cstr(str, rb_utf8_encoding());
+}
+#else
+static VALUE
+interned_utf8_cstr(const char * str)
+{
+    VALUE rb_str = rb_utf8_str_new_cstr(str);
+    rb_obj_freeze(rb_str);
+    rb_funcall(rb_str, rb_intern("-@"), 0);
+    return rb_str;
+}
+#endif
+
 /* call-seq: stmt.column_name(index)
  *
  * Get the column name at +index+.  0 based.
@@ -382,7 +399,7 @@ column_name(VALUE self, VALUE index)
     VALUE ret = Qnil;
 
     if (name) {
-        ret = rb_enc_interned_str_cstr(name, rb_utf8_encoding());
+        ret = interned_utf8_cstr(name);
     }
     return ret;
 }