configure.ac

Fix build when targeting closed src ftd2xx drivers.
configure is unable to find the dynamic linking loader lib (dl) as it
is included before ftd2xx library.

Change-Id: Ibe7308b66ed846288a31f7a27ff549b6f39baeec
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1355


Tested-by: jenkins
Reviewed-by: Luca Bruno <lucab@debian.org>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>