configure.ac: fix check for jimtcl submodule

configure.ac: fix check for jimtcl submodule

The file configure.ac checks if jimtcl submodule is present by
looking for the file jimtcl/configure.ac .
But jimtcl has switched to its own build system in 2011 and thus
dropped configure.ac . This cause a build issue on OpenOCD that
was incorrectly fixed by adding a dummy configure.ac in jimtcl.

Fix OpenOCD configure.ac to look for the correct file
jimtcl/configure .
After this fix, another fix would be proposed to jimtcl to drop
its useless dummy file configure.ac .

Change-Id: I705f72d83e374c8dd700baaa0c0bbe041f153605
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7437
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>