$NetBSD$

Avoid linking against non-existent libdl. From upstream, see
https://code.ros.org/trac/opencv/ticket/726

--- CMakeLists.txt.orig	2010-04-06 01:24:37.000000000 +0000
+++ CMakeLists.txt
@@ -438,7 +438,7 @@ if(UNIX)
             endif()
         endif()
         
-        if(${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD")
+        if(${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD" OR ${CMAKE_SYSTEM_NAME} MATCHES "NetBSD")
             set(OPENCV_LINKER_LIBS m pthread)
         else()
             set(OPENCV_LINKER_LIBS dl m pthread rt)