The problem us that gb_LinkTarget__use_curl only declared a dependency
on an *unpacked* curl tarball, but the curlbuild.h file is *generated*
during curl configury. So something that depends on a (non-system)
curl needs to depend on curl having been configured at least. Let's
try like this.

Change-Id: I87b2a3292807d9bb873c3656caf58c4d98d8f622