when running layout_test ensure that (#209)

layout tests are present, by generating bindings!

Cargo.toml

@@ -27,7 +27,7 @@ clang = { version = "2", optional = true, features = ["runtime", "clang_3_7"] }
 # Turn on the 'use-bindgen' feature to generate bindings on the fly for your platform.
 use-bindgen = ["bindgen", "clang"]
 # Enables generation of layout-tests in bindgen
-layout_tests = []
+layout_tests = ["use-bindgen"]
 
 [lib]
 # Some code comments on R's source code might be accidentally treated as Rust's