Allow specifying make via an environment variable.

This is what mozjs does, and allows me to fix up servo/webrender#3308.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "osmesa-src"
-version = "0.1.0"
+version = "0.1.1"
 authors = ["mesa3d maintainers"]
 build = "build.rs"
 repository = "https://github.com/servo/osmesa-src/"

build.rs

@@ -1,7 +1,19 @@
 use std::env;
+use std::ffi::{OsStr, OsString};
 use std::path::PathBuf;
 use std::process::Command;
 
+fn find_make() -> OsString {
+    if let Some(make) = env::var_os("MAKE") {
+        return make
+    }
+
+    match Command::new("gmake").status() {
+        Ok(_) => OsStr::new("gmake").to_os_string(),
+        Err(_) => OsStr::new("make").to_os_string(),
+    }
+}
+
 fn main() {
     let src = PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").unwrap());
     let dst = PathBuf::from(env::var_os("OUT_DIR").unwrap());
@@ -45,7 +57,7 @@ fn main() {
                 .arg("--enable-gallium-osmesa")
                 .arg("--with-gallium-drivers=swrast"));
 
-    run(Command::new("make")
+    run(Command::new(find_make())
                 .env("MAKEFLAGS", env::var("CARGO_MAKEFLAGS").unwrap_or_default())
                 .env("PYTHONPATH", src.join("Mako-1.0.7.zip"))
                 .current_dir(&dst));