file: Fix cross-compilation on Darwin/macOS

Darwin/macOS does not have pipe2(2).

Apply a similar guard as in f3d7ace4
after 43a5ec4e.

Pull Request: https://github.com/freebsd/freebsd-src/pull/574

(cherry picked from commit f5ccb3a3)

lib/libmagic/config.h

@@ -126,7 +126,9 @@
 #endif
 
 /* Define to 1 if you have the `pipe2' function. */
+#ifndef __APPLE__ /* Cross building tools on macOS */
 #define HAVE_PIPE2 1
+#endif
 
 /* Define to 1 if you have the `pread' function. */
 #define HAVE_PREAD 1