Add Mk/Uses/alias.mk

When on used FreeBSD, this tool doesn't do anything. However, when used
on DragonFly, it will define FreeBSD appropriately in CFLAGS which
allows the port to build with minimal or no extra patches.

Differential Revision: https://reviews.freebsd.org/D950
Reviewed by: portmgr
Approved by: bapt (portmgr)