hopenssl.cabal: add link-libz flag enable an explicit dependency on libz

Static variants of libcrypto may depend on libz but can't express that
dependency, so we have to worry about telling Cabal about it.

Relevant Issue:
 - NixOS/cabal2nix#389
 - haskell/cabal#5805

hopenssl.cabal

@@ -20,6 +20,11 @@ source-repository head
   type:     git
   location: git://github.com/peti/hopenssl.git
 
+flag link-libz
+  default:     False
+  manual:      True
+  description: OpenSSL\'s @libcrypto@ has a dependency on @libz@
+
 library
   exposed-modules:  OpenSSL.Digest
                     OpenSSL.EVP.Digest
@@ -41,6 +46,9 @@ library
   includes:         openssl/evp.h
   build-tools:      hsc2hs
 
+  if flag(link-libz)
+    extra-libraries: z
+
 test-suite check-low-level-digest-api
   type:           exitcode-stdio-1.0
   main-is:        CheckLowLevelDigestAPI.hs