add env_var output for cargo:rustc-env (see rust-lang/cargo#3929)

src/output.rs

@@ -10,6 +10,9 @@ output!(/// Pass a library search path to the compiler as a `-L` flag
 output!(/// Pass a cfg flag to the compiler
     fn cfg, "rustc-cfg", &str);
 
+output!(/// Set a variable in the compiler's environment
+    fn env_var, "rustc-env", &str, &str);
+
 output!(/// Specify a file or directory whose timestamp will trigger a rerun of the build script
         /// (note: to recursively track a directory, see `rerun_walk`)
     fn rerun, "rerun-if-changed", <&Path>);

tests/foo/build.rs

@@ -22,6 +22,7 @@ fn main() {
     link_lib(LibKind::Dylib, Path::new("whatever"));
     link_search(SearchKind::All, Path::new("wherever"));
     cfg("whoever");
+    env_var("FOO", "foo");
     rerun(Path::new("whichever"));
     rerun_walk(Path::new("src"), |_| true);
     warning("success");

tests/test.rs

@@ -39,6 +39,7 @@ fn test() {
 cargo:rustc-link-lib=dylib=whatever
 cargo:rustc-link-search=all=wherever
 cargo:rustc-cfg=whoever
+cargo:rustc-env=FOO=foo
 cargo:rerun-if-changed=whichever
 cargo:rerun-if-changed=src
 cargo:rerun-if-changed=src/lib.rs