fix zlib redundant crc32; add rust_backend to travis CI

Author: Frommi

.travis.yml

@@ -15,6 +15,8 @@ script:
   - cargo test --verbose --features tokio
   - cargo test --verbose --features 'tokio zlib'
   - cargo test --verbose --features zlib --no-default-features
+  - cargo test --verbose --features rust_backend
+  - cargo test --verbose --features rust_backend --no-default-features
   - cargo clean && cargo build
   - cargo doc --no-deps
   - cargo doc --no-deps --manifest-path=miniz-sys/Cargo.toml

src/ffi.rs

@@ -169,6 +169,7 @@ mod imp {
     }
 }
 
+#[cfg(not(feature = "zlib"))]
 mod crc_imp {
     use libc::{c_ulong, off_t};
     pub unsafe extern fn mz_crc32_combine(crc1: c_ulong,