Basic Cargo build

Builds the uutils multicall binary containing all utils (except stdbuf)
by default. To only build a subset
    `cargo --no-default-features --features <utils>`
can be used.

Whats missing is building the standalone binaries and a mechanism to
automatically disable the build of unix only utils on windows.

build.rs

@@ -0,0 +1,61 @@
+use std::env;
+use std::fs::File;
+use std::io::Write;
+use std::path::Path;
+
+pub fn main() {
+    let feature_prefix = "CARGO_FEATURE_";
+    let out_dir = env::var("OUT_DIR").unwrap();
+
+    let mut crates = Vec::new();
+    for (key, val) in env::vars() {
+        if val == "1" && key.starts_with(feature_prefix) {
+            let krate = key[feature_prefix.len()..].to_lowercase();
+            match krate.as_ref() {
+            "default" => continue,
+            "all" => continue,
+            _ => {},
+            }
+            crates.push(krate.to_string());
+        }
+    }
+    crates.sort();
+    let mut cf = File::create(Path::new(&out_dir).join("uutils_crates.rs")).unwrap();
+    let mut mf = File::create(Path::new(&out_dir).join("uutils_map.rs")).unwrap();
+    mf.write_all("
+    type UtilityMap = HashMap<&'static str, fn(Vec<String>) -> i32>;
+
+    fn util_map() -> UtilityMap {
+    let mut map: UtilityMap = HashMap::new();\n".as_bytes()).unwrap();
+    for krate in crates {
+        match krate.as_ref() {
+            "false" => continue,
+            "true" => continue,
+            _ => cf.write_all(format!("extern crate {krate} as uu{krate};\n", krate=krate).as_bytes()).unwrap(),
+        }
+
+        match krate.as_ref() {
+            "hashsum" => {
+                mf.write_all("map.insert(\"hashsum\", uuhashsum::uumain);
+                              map.insert(\"md5sum\", uuhashsum::uumain);
+                              map.insert(\"sha1sum\", uuhashsum::uumain);
+                              map.insert(\"sha224sum\", uuhashsum::uumain);
+                              map.insert(\"sha256sum\", uuhashsum::uumain);
+                              map.insert(\"sha384sum\", uuhashsum::uumain);
+                              map.insert(\"sha512sum\", uuhashsum::uumain);\n".as_bytes()).unwrap();
+            },
+            "false" =>
+                mf.write_all("fn uufalse(_: Vec<String>) -> i32 { 1 }
+                             map.insert(\"false\", uufalse as fn(Vec<String>) -> i32);\n".as_bytes()).unwrap(),
+            "true" =>
+                mf.write_all("fn uutrue(_: Vec<String>) -> i32 { 0 }
+                             map.insert(\"true\", uutrue as fn(Vec<String>) -> i32);\n".as_bytes()).unwrap(),
+            _ => 
+                mf.write_all(format!("map.insert(\"{krate}\", uu{krate}::uumain as fn(Vec<String>) -> i32);\n", krate= krate).as_bytes()).unwrap(),
+        }
+    }
+    mf.write_all("map
+    }\n".as_bytes()).unwrap();
+    cf.flush().unwrap();
+    mf.flush().unwrap();
+}