UnixPkg/UnixSnpDxe is currently BSD/OS X specific.
Therefore we only build it if "-D NETWORK_SUPPORT"
is used on the build command line.
UnixPkg/build.sh and UnixPkg/build64.sh are updated to
automatically define NETWORK_SUPPORT if building on OS X.
<net/if_dl.h> and <net/bpf.h> are also only included in
UnixPkg/Include/Common/UnixInclude.h if __APPLE__ is defined.
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@11346 6f19259b-4bc3-4df7-8a09-765794883524