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>