Xcode framework: USE_CURL in Debug

The docs state that MMD6 build without external dependencies, but in order to build the Xcode framework project directly using some generic iOS device as platform target, building `epub.h` fails because curl is not found on a vanilla machine. Removing the `USE_CURL` build argument helps, obviously.

I think it'd be a good idea to add a little notice to the install instructions, Wiki, or README about installing `curl`.

Edit: I notices that I do have curl installed via homebrew, but it doesn't seem to be part of the linkers search path by default. I'm not sure if this is something you have control over, how the `find_package(CURL); if (CURL_FOUND)` implementation is actually executed.