File ffcall/README.os2 artifact ab0137e5dd part of check-in ed10f826ff

Installation on OS/2:

- Requires emx+gcc and a working shell (Bourne shell, Korn shell or GNU bash).

- Set the environment variables CC=gcc CFLAGS="-Zexe" before calling
  `configure'.

- Must build in a separate directory and pass the --srcdir option to
  `configure', otherwise avcall-i386.s will overwrite avcall-i386.S.