lookups.
This uses a special variable name based on a hash of ${CC}, ${PATH}, and
${MACHINE} to ensure that a cached value is not used if any of these
values changes to use a new compiler.
Before this there were 34,620 fork/exec from bsd.compiler.mk during a buildworld.
After this there are 608. More improvement is needed to cache a value from
the top-level before descending into subdirs in the various build phases.
Reviewed by: brooks (earlier version)
MFC after: 3 weeks
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D3898
c197d1a9f7