add gnustep-opal

In darling project, src/dyld/CMakeLists.txt :
~~~~
if (DEBUG)
	set(dyld_SRCS
		${dyld_SRCS}
		UndefinedFunction.cpp
	)

endif (DEBUG)
~~~~

cause an error when linking dyld:
~~~~
darling/src/dyld/MachOMgr.cpp:(.text+0x1ba9): undefined reference to `UndefMgr::~UndefMgr()'
darling/src/dyld/MachOMgr.cpp:(.text+0x1be3): undefined reference to `UndefMgr::UndefMgr(int)'
~~~~