build: Fix apple-non-apple cross building

When cross compiling for an apple platform on a non-apple platform
(or the reverse), `if(APPLE)` in line 32 should refer to the target
platform. Without importing the `APPLE` variable, it refers to the
host platform.

Reviewed-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>