disable use of libraries/dependencies from system in SuperBuild
USE_SYSTEM_* variables from superbuild. There are around 35 libraries used in superbuild now. Allowing them to use an existing install of this library can create more problems. I don't remember the exact motive behind this design decision. My guess, is for debugging purposes. It is so easy to screw things up. It "must" work on all platforms and have same number of tests passing.
Anyway there is no way to test all combination of system v. superbuild of all libraries in OTB. Even if it can "argued" to doable that falls right on sane/insane check. Testing all combination will explode any of existing build matrix ever available for a single project in history. Btw, remember we have 2 platforms and at-least 5-10 linux distribution to test each combination.
With this feature done, superbuild can be tested on maximum number of platforms and can fix serious bugs come out of it. I believe this would be a nice deal for project.
Regarding big libraries, its not taking too much time. After all, you opted to use superbuild for a cause which sure has its price/