helper/command: fix build with jimtcl 0.79 or older

helper/command: fix build with jimtcl 0.79 or older

Commit a7d68878e4ba ("helper/command: unregister commands through
their full-name") introduces for the first time in OpenOCD the use
of jimtcl API Jim_DeleteCommand().
The prototype of Jim_DeleteCommand() has changed with jimtcl 0.80
and the current code doesn't build with jimtcl 0.79 or older. This
is an issue for those distributions, like Debian, that provide
jimtcl as a separate package/library and have not switched yet to
the new jimtcl version.

Add a compile-time condition to cope with the jimtcl API change.

Change-Id: Ic813ab7c0ebd3c8772f27775ba3912a47d5c275c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: a7d68878e4ba ("helper/command: unregister commands through their full-name")
Reviewed-on: http://openocd.zylin.com/6191
Tested-by: jenkins