configure.ac: add libutil to the dependency list

configure.ac: add libutil to the dependency list

Jimtcl 0.80 (2020-10-29) adds dependency to libutil, which is part
of the GNU libc.
The library is searched and used by jimtcl build, but still has to
be linked in by OpenOCD as indirect dependency.

While OpenOCD is still using jimtcl 0.79, let's prepare to switch
to the next version.
Add libutil search in configure.ac.

Change-Id: I5a8952161b0e5b93fc00d23be256b5274d31e08c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5922
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>