MFC r312332,r312446,r312451:

r312332:

Use SRCTOP where possible and use :H to manipulate .CURDIR to get rid of
unnecessarily long relative path .PATH values with make

r312446 (by emaste):

libc: remove reference to nonexistent lib/locale directory

As far as I can tell this was introduced in r72406 and updated in several
subsequent revisions, but the lib/locale directory it referenced never
existed.

r312451:

Replace dot-dot relative pathing with SRCTOP-relative paths where possible

This reduces build output, need for recalculating paths, and makes it clearer
which paths are relative to what areas in the source tree. The change in
performance over a locally mounted UFS filesystem was negligible in my testing,
but this may more positively impact other filesystems like NFS.

LIBC_SRCTOP was left alone so Juniper (and other users) can continue to
manipulate lib/libc/Makefile (and other Makefile.inc's under lib/libc) as
include Makefiles with custom options.

Discussed with: marcel, sjg