make(1): Create two more custom variables to speed up dports
I've added two new static values to bmake:
.MAKE.DF.OSREL
This will currently return "4.5"
.MAKE.DF.VERSION
This will currently return "400500". It matches what was in place on
/usr/src/sys/sys/param.h during the last buildworld.
Every dport runs piped shell commands to get these values. After iterating
through the entire ports tree several times, I noticed a large presence
of awk in process list. After modifying bmake and then modifying dports
locally, the iteration runs 15% faster. There are other optimizations
that can be done, but that's out of scope of this commit.