Add a feature to explicitly enable "std"

If you know you need "std", e.g. relying on default `S = RandomState`,
then you can enable this feature to force it on, bypassing target
detection. This may help scenarios where `autocfg` fails detection, like
issue indexmap-rs#144, but at the very least it will speed up the build script by
avoiding extra `rustc` invocations.

.travis.yml

@@ -10,6 +10,9 @@ matrix:
     - rust: stable
       env:
        - FEATURES='rayon'
+    - rust: stable
+      env:
+       - FEATURES='std'
     - rust: beta
     - rust: nightly
       env:

Cargo.toml

@@ -53,6 +53,9 @@ fxhash = "0.2.1"
 # Serialization with serde 1.0
 serde-1 = ["serde"]
 
+# Force the use of `std`, bypassing target detection.
+std = []
+
 # for testing only, of course
 test_low_transition_point = []
 test_debug = []

build.rs

@@ -1,5 +1,8 @@
 fn main() {
-    let ac = autocfg::new();
-    ac.emit_sysroot_crate("std");
+    // If "std" is explicitly requested, don't bother probing the target for it.
+    match std::env::var_os("CARGO_FEATURE_STD") {
+        Some(_) => autocfg::emit("has_std"),
+        None => autocfg::new().emit_sysroot_crate("std"),
+    }
     autocfg::rerun_path("build.rs");
 }