The env space consumed by exporting all libc's .meta files
left little room for command line,
so unexport when done.
Update dirdeps.mk to latest and add
dirdeps-targets.mk to simplify/update targets/Makefile
Makefile changes to go with Makefile.depend changes in D22494
Reviewed by: bdrewery
MFC after: 1 week
Sponsored by: Juniper Networks
Differential Revision: https://reviews.freebsd.org/D22495
