Due to consideration of POSIX compliance issues config.mk had
to be removed. Configuration variables can be overridden by
environment variables or specified via command line.

Additionally all pretty-printed messages were removed and
built-in rules are used. This also simplifies and purges
the Makefile.

Ptm and pts are used by pty.c, thus they have to be non-static.

Std requires I/O multiplexing which is very complicated with
stdio, because it provides no transparent buffering.

The xinerama related variables in config.mk are a relict of
dwm and therefore should be removed.

Proprietary UNIX variants like AIX, HP-UX or SCO UNIX
are nowadays rarely used and maintaining compatibility to
non-standard operating systems interfaces is wasteful and
practically useless.

Projects like xterm suffer from backward compatibility to decades-old
UNIX variants and terminal standards. This does not conform to st's
design goals.