Drop bundled zlib

We're not doing a sufficient job of keeping this updated so list
it as an external requirement instead.
diff --git a/cmake/StaticBuild.cmake b/cmake/StaticBuild.cmake
index 49706bd..bac2202 100644
--- a/cmake/StaticBuild.cmake
+++ b/cmake/StaticBuild.cmake
@@ -15,7 +15,7 @@
 
   set(JPEG_LIBRARIES "-Wl,-Bstatic -ljpeg -Wl,-Bdynamic")
 
-  if(WIN32 AND NOT USE_INCLUDED_ZLIB)
+  if(WIN32)
     set(ZLIB_LIBRARIES "-Wl,-Bstatic -lz -Wl,-Bdynamic")
   endif()