Clarifying unix-like support

Added `build.rs` to  produce a compile error when attempting to build on a
non-Unix-like system, and clarified support in `readme.md`.

Signed-off-by: Jonathan Woollett-Light <jcawl@amazon.co.uk>

README.md

@@ -7,6 +7,8 @@ implementation is stateless thus it does not support chunking or compression.
 
 The micro-http implementation is used in production by Firecracker.
 
+As micro-http uses [`std::os::unix`](https://doc.rust-lang.org/std/os/unix/index.html) this crates only supports Unix-like targets.
+
 ## Contributing
 
 To contribute to micro-http, checkout the

build.rs

@@ -0,0 +1,4 @@
+fn main() {
+    #[cfg(not(target_family = "unix"))]
+    std::compile_error!("This crate only supports Unix-like targets");
+}