Remaining sa2 warnings under macOS

[sh95014]

After #343, there are still a couple of warnings:

[ 47%] Building CXX object source/CMakeFiles/appleii.dir/linux/network/slirp2.cpp.o
/Users/sh95014/Developer/Mariani/source/linux/network/slirp2.cpp:181:5: warning: 'slirp_pollfds_fill' is deprecated [-Wdeprecated-declarations]
  181 |     slirp_pollfds_fill(mySlirp.get(), &timeout, net_slirp_add_poll, this);
      |     ^
/opt/homebrew/Cellar/libslirp/4.9.1/include/slirp/libslirp.h:274:64: note: 'slirp_pollfds_fill' has been explicitly marked deprecated here
  274 |                         SlirpAddPollCb add_poll, void *opaque) SLIRP_DEPRECATED;
      |                                                                ^
/opt/homebrew/Cellar/libslirp/4.9.1/include/slirp/libslirp.h:40:41: note: expanded from macro 'SLIRP_DEPRECATED'
   40 | #define SLIRP_DEPRECATED __attribute__((__deprecated__))
      |                                         ^
1 warning generated.

This looks like a real update of slirp2.cpp to move to the socket-based API, so I left it alone.

[100%] Linking CXX executable ../../../sa2
ld: warning: ignoring duplicate libraries: '../../libappleii.a'
[100%] Built target sa2

This looks like libappleii.a got listed twice in the call to ld, but I'm not really familiar enough with CMake to really dig into it.