Include zlib linker flag if not building with the vendored one

Apparently building with LLVM enabled includes this flag, probably
because of using `--system-libs`, so building without LLVM fails because
it can't link against zlib.

rakelib/blueprint.rb

@@ -179,6 +179,8 @@
     end
     gcc.add_library zlib
     files << zlib
+  else
+    gcc.ldflags << "-lz"
   end
 
   if Rubinius::BUILD_CONFIG[:vendor_libsodium]