compile: gracefully fail if use_std is not used

Use `compile_error!` macro to issue a meaningful compile
error if the `use_std` feature is not enabled.

Fixes #481

src/lib.rs

@@ -521,6 +521,11 @@ another matching engine with fixed memory requirements.
 #![cfg_attr(test, deny(warnings))]
 #![cfg_attr(feature = "pattern", feature(pattern))]
 
+
+#[cfg(not(feature = "use_std"))]
+compile_error!("`use_std` feature is currently required to build this crate");
+
+
 extern crate aho_corasick;
 extern crate memchr;
 extern crate thread_local;