cmake: enable policy CMP0042

Enable CMake policy CMP0042, if supported:

> CMake 2.8.12 and newer has support for using ``@rpath`` in a target's
> install name.  This was enabled by setting the target property
> ``MACOSX_RPATH``.  The ``@rpath`` in an install name is a more
> flexible and powerful mechanism than ``@executable_path`` or
> ``@loader_path`` for locating shared libraries.

CMakeLists.txt

@@ -17,6 +17,9 @@ CMAKE_POLICY(SET CMP0015 NEW)
 IF (POLICY CMP0051)
 	CMAKE_POLICY(SET CMP0051 NEW)
 ENDIF()
+IF (POLICY CMP0042)
+	CMAKE_POLICY(SET CMP0042 NEW)
+ENDIF()
 
 # Add find modules to the path
 SET(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${libgit2_SOURCE_DIR}/cmake/Modules/")