Use posix_spawn if possible.

Use posix_spawn if possible.

posix_spawn is less error-prone than vfork + execve, and can make
better use of system-specific enhancements like 'clone' on Linux.  Use
it if we don't need to configure a pseudoterminal.

* src/Makefile.in (LIB_POSIX_SPAWN): New variable.
(LIBES): Use it.

* src/callproc.c (emacs_spawn): Use posix_spawn on Unix-like system if
we don't need to set up a pseudoterminal.