Rust: defaults to the native style when Qt is available

This requires some gymnastics to get right as the information
need to be passed to the compiler despite having no direct dependency
between the compiler and the runtime or backends.
So use a file in the build directory to tell the default style

cc: #83

api/sixtyfps-rs/sixtyfps-macros/lib.rs

@@ -340,6 +340,13 @@ pub fn sixtyfps(stream: TokenStream) -> TokenStream {
     //println!("{:#?}", syntax_node);
     let mut compiler_config =
         CompilerConfiguration::new(sixtyfps_compilerlib::generator::OutputFormat::Rust);
+
+    if std::env::var_os("SIXTYFPS_STYLE").is_none() {
+        compiler_config.style = std::fs::read_to_string(env!("SIXTYFPS_DEFAULT_STYLE_PATH"))
+            .map(|style| style.trim().into())
+            .ok()
+    }
+
     compiler_config.include_paths = include_paths;
     let (root_component, mut diag) =
         spin_on::spin_on(compile_syntax_node(syntax_node, diag, compiler_config));