Try system dylib when compiling for Apple platforms

Fixes #84

Author: kornelski

build.rs

@@ -67,10 +67,13 @@ fn main() {
     // MSVC basically never has it preinstalled, MinGW picks up a bunch of weird
     // paths we don't like, `want_static` may force us, and cross compiling almost
     // never has a prebuilt version.
+    //
+    // Apple platforms have libz.1.dylib, and it's usually available even when
+    // cross compiling (via fat binary or in the target's Xcode SDK)
     if target.contains("msvc")
         || target.contains("pc-windows-gnu")
         || want_static
-        || target != host
+        || (target != host && !target.contains("-apple-"))
     {
         return build_zlib(&mut cfg, &target);
     }