ssh: Move XAUTH_PATH setting to ssh.mk

XAUTH_PATH is normally set (in the upstream build infrastructure) in
config.h. We previously set it in ssh and sshd's Makefiles if LOCALBASE
is set, and over time have sometimes also defined it in config.h.

Leave it unset in config.h and move the CFLAGS logic to to ssh.mk so
that it will be set when building all ssh libraries and programs but
still be set by LOCALBASE.

Reviewed by: jlduran
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48907

(cherry picked from commit a63701848fe5462c4e8bbff0131bb42979e603ec)