COMP: Remove extra calls to cmake_minimum_required

Drop calls to this command in subdirectories.  They are never used as
the top-level source directory.

Change-Id: I36d9447c424ca8a8d0daa6ae028ca1cb66eb68c2
COMP: Fix cmake_minimum_required call order

Call the command first in any given CMakeLists.txt file.  While at it,
update some calls to match ITK's current minimum of CMake 2.8.6 and drop
old policy settings accordingly.

Change-Id: Ice9bcab71494659b0e4840ce70b2500ccc929acd
COMP: Set CMake Policies CMP0025 and CMP0042 as necessary

CMake 3.0 introduces these policies to improve default behavior on OS X.
Set them explicitly in ITK to avoid policy warnings.  ITK already works
with the NEW behavior.

Policy CMP0042 affects shared libraries on OS X, enabling MACOSX_RPATH
by default.  Set it in directories where we create libraries.  Also tell
KWSys to enable MACOSX_RPATH explicitly instead of setting the policy.

Change-Id: I3411ede1ee88c62c8d10bec101740f5e5e23e55d
Merge topic 'fix-CMP0025-CMP0042'

60974eae COMP: Set CMake Policies CMP0025 and CMP0042 as necessary
51b4ccc6 COMP: Fix cmake_minimum_required call order
1d5fe0b7 COMP: Remove extra calls to cmake_minimum_required