* configure.host: Disable new posix_spawn function for all
users of posix dir except Cygwin.
* libc/posix/Makefile.am: Add support for new posix_spawn function.
* libc/posix/Makefile.in: Regenerate.
* libc/posix/posix_spawn.c: New file.
* libc/include/spawn.h: Ditto.
Jeff Johnston