Use NetBSD to initialize OpenBSD configuration

We teach Modules/Platform/OpenBSD.cmake to load NetBSD first since the
platforms are so similar.  This enables RPATH support on OpenBSD.

Modules/Platform/OpenBSD.cmake

@@ -1,3 +1,2 @@
-SET(CMAKE_DL_LIBS "")
+INCLUDE(Platform/NetBSD)
 SET_PROPERTY(GLOBAL PROPERTY FIND_LIBRARY_USE_OPENBSD_VERSIONING 1)
-INCLUDE(Platform/UnixPaths)