build: create group features for target systems

This allows a user to create builds with or without the Unix-specific
utilities. Right now the Makefile handles this, but this is needed to
migrate away from make.

To build Unix-specific utilities (default):

cargo build
(or)
cargo build --features unix --no-default-features

To build without the Unix-specific utilities:

cargo build --features generic --no-default-features

build.rs

@@ -12,8 +12,7 @@ pub fn main() {
         if val == "1" && key.starts_with(feature_prefix) {
             let krate = key[feature_prefix.len()..].to_lowercase();
             match krate.as_ref() {
-            "default" => continue,
-            "all" => continue,
+            "default" | "unix" | "generic" => continue,
             _ => {},
             }
             crates.push(krate.to_string());