Set CXXFLAGS to include $VERSION_MIN_FLAGS.

You never know when some library might start using C++, so set CXXFLAGS
everywhere.

Change-Id: I7e49a3446bb6b587e20058be48e06a5f345679c3
Reviewed-on: https://code.wireshark.org/review/5036
Reviewed-by: Guy Harris <guy@alum.mit.edu>