fix(dependencies): use working version of serde-codegen

This update fixes the build on stable, and allows builds
on nightly as usual.

The trick is to use the latest version of serde-codegen,
which keeps the syntex version internal, preventing clashes
between libraries that might have different requirements.

src/mako/api/build.rs.mako

@@ -1,7 +1,5 @@
-<%namespace name="util" file="../lib/util.mako"/>\
-#[cfg(feature = "with-syntex")]
+#[cfg(feature = "with-serde-codegen")]
 mod inner {
-    extern crate syntex;
     extern crate serde_codegen;
 
     use std::env;
@@ -13,13 +11,11 @@ mod inner {
         let src = Path::new("src/lib.rs.in");
         let dst = Path::new(&out_dir).join("lib.rs");
 
-        let mut registry = syntex::Registry::new();
-        serde_codegen::register(&mut registry);
-        registry.expand("${util.crate_name()}", &src, &dst).unwrap();
+        serde_codegen::expand(&src, &dst).unwrap();
     }
 }
 
-#[cfg(not(feature = "with-syntex"))]
+#[cfg(not(feature = "with-serde-codegen"))]
 mod inner {
     pub fn main() {}
 }

src/mako/api/lib.rs.mako

@@ -24,5 +24,5 @@ ${lib.docs(c)}
 #[cfg(feature = "nightly")]
 include!("lib.rs.in");
 
-#[cfg(feature = "with-syntex")]
+#[cfg(feature = "with-serde-codegen")]
 include!(concat!(env!("OUT_DIR"), "/lib.rs"));