libsepol,checkpolicy,secilc: Replace #ifdef DARWIN with __APPLE__.

As per discussion in https://android-review.googlesource.com/#/c/221980,
we should be using #ifdef __APPLE__ rather than our own custom-defined
DARWIN for building on MacOS X.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
9 files changed