Only specify the dependencies if we are making the copy target.

disable the rule to copy resize.c since it fails on r/o src trees:

cp: /home/source/ab/HEAD/src/usr.bin/resize/resize.c: Permission denied

no need to specify SRCS

Don't define the copy rules if we don't have X sources

Bump warns

Add a maintainable version of resize, with a copy target and a header that
provides all the glue needed without other X headers.

Import Xterm's resize(1) for querying (x)terminal sizes in base for headless
clients