fix CI by assuring builds work with --all-features enabled (#404)

Author: Byron

src/ffi/c.rs

@@ -403,7 +403,7 @@ mod c_backend {
     #[cfg(feature = "zlib-ng")]
     use libz_ng_sys as libz;
 
-    #[cfg(feature = "zlib-rs")]
+    #[cfg(all(not(feature = "zlib-ng"), feature = "zlib-rs"))]
     use libz_rs_sys as libz;
 
     #[cfg(all(not(feature = "zlib-ng"), feature = "cloudflare_zlib"))]
@@ -443,7 +443,7 @@ mod c_backend {
 
     #[cfg(feature = "zlib-ng")]
     const ZLIB_VERSION: &'static str = "2.1.0.devel\0";
-    #[cfg(feature = "zlib-rs")]
+    #[cfg(all(not(feature = "zlib-ng"), feature = "zlib-rs"))]
     const ZLIB_VERSION: &'static str = "0.1.0\0";
     #[cfg(not(any(feature = "zlib-ng", feature = "zlib-rs")))]
     const ZLIB_VERSION: &'static str = "1.2.8\0";