Support multiple --no-unstable-rust options

Dirty hack to pass legacy test suite.

Author: jdub

src/lib.rs

@@ -195,7 +195,8 @@ pub fn builder_from_flags<I>(args: I) -> Result<(Builder, Box<io::Write>), io::E
                 .help("Don't automatically convert floats to f32/f64."),
             Arg::with_name("no-unstable-rust")
                 .long("no-unstable-rust")
-                .help("Do not generate unstable Rust code."),
+                .help("Do not generate unstable Rust code.")
+                .multiple(true), // FIXME: Pass legacy test suite
             Arg::with_name("opaque-type")
                 .long("opaque-type")
                 .help("Mark a type as opaque.")