Set CMP0042 explicitly with CMake 3.0 and above

Some host projects no longer set `CMAKE_MACOSX_RPATH` explicitly due to
requiring CMake 3.0 or above, and now see CMP0042 warnings.  Set the
policy explicitly in KWSys to avoid the warnings.  Host projects can
still set `CMAKE_MACOSX_RPATH` to `0` to get the old behavior.

Suggested-by: Ken Martin <ken.martin@kitware.com>

CMakeLists.txt

@@ -84,6 +84,7 @@
 CMAKE_MINIMUM_REQUIRED(VERSION 2.6.3 FATAL_ERROR)
 FOREACH(p
     CMP0025 # CMake 3.0, Compiler id for Apple Clang is now AppleClang.
+    CMP0042 # CMake 3.0, MACOSX_RPATH is enabled by default.
     CMP0048 # CMake 3.0, Let the project command manage version variables.
     CMP0056 # CMake 3.2, Honor link flags in try_compile() source-file signature.
     CMP0063 # CMake 3.3, Honor visibility properties for all target types.