Add OS/2 support

    modified:   ../../.gitignore
    modified:   Makefile
    new file:   mk.cmd

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Date:      Tue Jan 12 13:15:45 2016 +0900
#
# On branch 1.4.0-os2
# Changes to be committed:
#	modified:   ../../.gitignore
#	modified:   Makefile
#	new file:   mk.cmd
#
# Untracked files:
#	gmake.log
#	mk.bak
#
