Refactor ccompile.h to not include system headers

This is a step toward being able to vendor the OpenZFS code in FreeBSD.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10625

include/sys/zfs_context.h

@@ -335,6 +335,9 @@ extern void cv_broadcast(kcondvar_t *cv);
 #define	tsd_set(k, v) pthread_setspecific(k, v)
 #define	tsd_create(kp, d) pthread_key_create((pthread_key_t *)kp, d)
 #define	tsd_destroy(kp) /* nothing */
+#ifdef __FreeBSD__
+typedef off_t loff_t;
+#endif
 
 /*
  * kstat creation, installation and deletion