Made Jam compilable on OS/2 with Watcom compiler.
* Makefile: Added appropriate section (you still have to use 'nmake'
with this Makefile, not the Watcom 'wmake')
* execunix.c: Fixed execcmd() for #ifdef OS_OS2. The temporary script
file is named .cmd instead of .bat so that an OS/2 shell will be
executed, not a DOS shell.